Multilevel storage nonvolatile semiconductor memory device enabling high-speed data reading and high-speed data writing

ABSTRACT

A nonvolatile semiconductor memory device transmits/receives data to/from a data input/output terminal every j bits (e.g., eight bits). Each of memory cells in a memory cell array can hold data of n bits in correspondence to 2 n  threshold levels. A write data conversion circuit generates write data from bit data input from the same data input/output terminal in a set of a plurality of data of j bits input at different timings.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nonvolatile semiconductor memorydevice, and more particularly, it relates to the structure of anonvolatile semiconductor memory device capable of storing informationof at least four values (information of at least two bits) in a singlememory cell. More specifically, the present invention relates to thestructure of an electrically reloadable nonvolatile semiconductor memorydevice such as a flash memory, for example.

2. Description of the Prior Art

In order to meet increase of the storage capacity of a nonvolatilesemiconductor memory device such as a flash memory, a structure capableof storing multivalued data exceeding binary data in a single memorycell has been developed.

FIG. 118 is a schematic block diagram showing the overall structure of aconventional AND flash memory 8000.

A memory cell array 100 includes a number of memory cells havingfloating gates and control gates. Referring to FIG. 118, the memory cellarray 100 is divided into two memory cell blocks 100R and 100L.

The control gates, drains and sources of the memory cells are connectedto word lines WL, bit lines BL and source lines SCL (not shown)respectively.

FIG. 118 representatively shows a single word line WL and a single bitline BL. Row decoders 110 selectively drive the word lines WL on thebasis of externally supplied address signals. A sense latch circuit 120is provided on single ends of the bit lines BL. The bit lines BL areselected on the basis of selection signals output from column decoders130, for transferring read data and write data.

The sense latch circuit 120 includes a column switching circuit (notshown in FIG. 118) for selecting the bit lines BL on the basis of theselection signals from the column decoders 130.

An address buffer 140 supplies the address signals to the columndecoders 130 and the row decoders 110.

A chip control part 200 externally receives an access control signal(not shown) and a clock signal (not shown) and entirely controls theinternal circuits of the flash memory 8000 for write control and readcontrol of the memory cells. The chip control part 200 controls a powersupply generation part 150 thereby switching operating voltages of worddrivers (not shown) driving the potentials of the word lines WL inresponse to operation modes for erasing, writing, reading and the like.

Data latch circuits DL-L and DL-R are data buffers temporarily holdingdata transferred in data write and read operations.

The operation modes of the flash memory 8000 are not particularlyrestricted but instructed by an access control signal externallysupplied to the chip control part 200 or command data supplied through adata bus or the like, and include data rewrite (erase and write) anddata read modes.

In the conventional AND four-valued flash memory 8000 shown in FIG. 118,each memory cell is in an information storage state selected from anerased state, a first write state, a second write state and a thirdwrite state. The four information storage states in total correspond tostates decided by 2-bit data. In other words, each memory cell can store2-bit data.

Therefore, the flash memory 8000 sets three different types of writeverify voltages applied to the word lines WL in the write operation andsequentially switches the voltages for performing write operations threetimes.

In each write operation, the chip control part 200 controls an operationof writing two-valued (1-bit) write data “0” or “1” (“L” or “H”) held ina sense latch SL (included in the sense latch circuit 120) connectedwith a memory cell subjected to writing while setting the correspondingwrite verify voltage every write operation. Information of four values(two bits) can be written in a single memory cell due to this structure,as described later in detail.

The flash memory 8000 sets three types of voltages as word lineselection levels applied by the word line WL in the read operations andcaptures data of two values (one bit) read from the memory cell in thethree read operations through the sense latch circuit 120, and the chipcontrol part 200 converts the data to information of four values (twobits) after termination of the three read operations.

The outlines of write and read operations are now described.

In the write operation, a data string of two values (one bit) to bewritten and address signals are captured in the address buffer/datainput/output buffer 140 from a data input/output terminal group 10 andan address signal input terminal group-12 respectively.

The chip control circuit 200 separates the data string of two values(one bit) to be written into data strings of upper and lower bit data(or data strings of odd and even bit data) and transfers the datastrings to the data latches (hereinafter referred to as non-selectedselection latches) DL-L and DL-R connected with non-selected memorycells in the memory cell array 100 through signal lines 20 respectivelyfor temporarily latching the data strings.

The chip control part 200 captures the data held in the data latchesDL-L and DL-R through the signal lines 20 when performing each of “write1 (write operation for obtaining the first write state)”, “write 2(write operation for obtaining the second write state)” and “write 3(write operation for obtaining the third write state)” and converts thedata to data “0” or “1” of two values (one bit) corresponding to thedata of four values (two bits) to be written in the selected memory cellin response to “write 1”, “write 2” and “write 3”. Further, the chipcontrol part 200 transfers the converted data to the sense latch SL inthe sense latch circuit 120 connected with the selected memory cellthrough a signal line 18 so that the aforementioned write operations“write 1”, “write 2” and “write 3” are performed in accordance with thebinary data latched in the selected sense latch SL.

Thus, information of four values (two bits) can be written in a singlememory cell by temporarily holding binary data separated into an upperbit string and a lower bit string in the data latches DL-L and DL-R,forming write data of two values (one bit) for each of three writeoperations (“write 1” to “write 3”) having different verify voltages andperforming the three write operations having different verify voltages.

In the read operation, three different types of voltages aresequentially applied to the selected word line WL so that information“0” or “1” of two values (one bit) read from the memory cell of thememory cell array 100 to the selected sense latch SL by three readoperations is transferred to and temporarily held in the data latchesDL-L and DL-R. Three types of data “0” or “1” of two values (one bit) inthe data strings read through the three read operations, held in thedata latches DL-L and DL-R and latched in the selected sense latch SLare transferred to the chip control circuit 200 through the signal lines18 and 20.

The chip control circuit 200 composites upper and lower bits of data offour values (two bits) on the basis of the data transferred in theaforementioned manner. The chip control circuit 200 outputs thecomposited upper and lower bits from the data input/output terminalgroup 10 through the data input/output buffer 140.

The aforementioned write and read operations are now described infurther detail.

[Conventional Write Operation of Four-Valued Data]

FIG. 119 illustrates the relation between write data and thresholds ofmemory cell transistors in a conventional two-valued AND flash memory.In the write and read operations, data are written and read withreference to a determination level Vj01.

FIG. 120 illustrates the relation between write data and thresholds ofmemory cell transistors in the conventional four-valued AND flash memory8000. In the write and read operations, data are written and read withreference to three determination levels Vj1, Vj2 and Vj3.

As hereinabove described, the conventional four-valued AND flash memory8000 divides the threshold into four types after writing as shown inFIG. 120, while the conventional two-valued AND flash memory divides thethreshold (Vth) of the memory cell transistor into two types “0” and “1”after writing.

Therefore, the flash memory 8000 requires three types of determinationlevels Vj1, Vj2 and Vj3 for determining the respective levels.

FIGS. 121 to 126 are conceptual diagrams showing data held in the datalatches DL-L and DL-R and the sense latch SL and thresholds of memorycells after writing in first to third processing steps of the writeoperation.

FIG. 121 shows data held in the latches DL-L, DL-R and SL in the firstprocessing step of the write operation, and FIG. 122 shows thresholds ofthe memory cells in the first processing step of the write operation.

FIG. 123 shows data held in the latches DL-L, DL-R and SL in the secondprocessing step of the write operation, and FIG. 124 shows thresholds ofthe memory cells in the second processing step of the write operation.

FIG. 125 shows data held in the latches DL-L, DL-R and SL in the thirdprocessing step of the write operation, and FIG. 126 shows thresholds ofthe memory cells in the third processing step of the write operation.

Before starting the write operation, the thresholds of the memory cellsare set below the determination level Vj1.

Referring to FIGS. 121 and 122, data DQ0 to DQ3 and DQ4 to DQ7 arestored in the data latches DL-R and DL-L respectively among data DQ0 toDQ7 for one byte input from terminals I/O0 to I/O7 included in the datainput/output terminal group 10 in the first step of the write operation.Referring to FIG. 121, it is assumed that C9 h represents the input1-byte data in hexadecimal notation.

The data latch DL-R latches the input data DQ0 to DQ3 from the terminalsI/O0 to I/O3 and the data latch DL-L latches the input data DQ4 to DQ7from the terminals I/O4 to I/O7 as for the entire sector (datacorresponding to one word line WL).

In the following description, consider each 2-bit data (DQ4, DQ0), (DQ5,DQ1), (DQ6, DQ2) and (DQ7, DQ3), having one of the data DQ4 to DQ7 heldin the data latch DL-L as the upper bit and one of the data DQ0 to DQ3held in the data latch DL-R as the lower bit, as a set of data.

The chip control part 200 operates the aforementioned sets of dataincluded in the data latches DL-R and DL-L and zeros only bit data ofthe sense latch SL corresponding to such data that the upper bit held inthe data latch DL-L is “0” and the lower bit held in the data latch DL-Ris “1”.

As shown in FIG. 121, the sense latch SL holds “0111” from thehigh-order position. On the basis of the data thus held in the senselatch SL, data are written in memory cells MC1 to MC4 corresponding tothe bits of the sense latch SL respectively. The memory cells MC1 to MC4are connected with the same word line WL. The third determination levelVj3 is employed as the determination value for a verify operation.

At this time, data is written in the memory cell corresponding to thedata “0” in the sense latch SL. Thus, the data (corresponding to data“01”) of level 4 is written in the memory cell MC4 corresponding to themost significant bit of the sense latch SL.

In practice, a high voltage is applied to the word line WL therebywriting the data through an FN (Fowler-Nordheim) tunnel current.

A voltage below the word line voltage is applied to bit lines BLcorresponding to the bit data “1” of the sense latch SL, in order torelax the voltage applied from the word line WL. Consequently, data iswritten in only the memory cell connected with the bit line BLcorresponding to the bit data “0” held in the sense latch SL.

Referring to FIGS. 123 and 124, the data held in the data latches DL-Rand DL-L are operated in the second step of the write operation forwriting “0” in the bit of the sense latch SL corresponding to such a setof data that the upper bit held in the data latch DL-L is “0” and thelower bit held in the data latch DL-R is “0”. The determination value inthe verify operation is changed to Vj2, and data are written only in thememory cells connected with bit lines BL corresponding to the data DQ5and DQ1 in data writing.

Referring to FIGS. 125 and 126, the data held in the data latches DL-Rand DL-L are operated in the third step of the write operation forwriting “0” in the bit of the sense latch SL corresponding to such a setof data that the upper bit held in the data latch DL-L is “1” and thelower bit held in the data latch DL-R is “0”. The determination value inthe verify operation is changed to Vj1, and data are written only in thememory cells connected with bit lines BL corresponding to the data DQ6and DQ2 in data writing.

After inputting all data to be written, the write operation isterminated through three operations and write processing as describedabove.

[Conventional Read Operation of Four-Valued Data]

The read operation is now described.

FIGS. 127 to 132 are conceptual diagrams showing data held in the datalatches DL-L and DL-R and the sense latch SL, thresholds of memory cellsand determination levels in first to third processing steps of the readoperation.

FIG. 127 shows data held in the latches DL-L, DL-R and SL in the firstprocessing step of the read operation, and FIG. 128 shows thresholds ofthe memory cells and determination levels in the first processing stepof the read operation.

FIG. 128 shows data held in the latches DL-L, DL-R and SL in the secondprocessing step of the read operation, and FIG. 129 shows threshold ofthe memory cells and determination levels in the second processing stepof the read operation.

FIG. 129 shows data held in the latches DL-L, DL-R and SL in the secondprocessing step of the read operation, and FIG. 130 shows thresholds ofthe memory cells and determination levels in the second processing stepof the read operation.

FIG. 131 shows data held in the latches DL-L, DL-R and SL in the thirdprocessing step of the read operation, and FIG. 132 shows thresholds ofthe memory cells and determination levels in the third processing stepof the read operation.

Referring to FIGS. 127 and 128, data are read at the first determinationlevel Vj1 and the results are stored in the sense latch SL in the firstprocessing step of the read operation. The data are transferred to thedata latch DL-R, and the sense latch SL is cleared.

Referring to FIGS. 129 and 130, data are read at the seconddetermination level Vj2 and the results are stored in the sense latch SLin the second processing step of the read operation. The data aretransferred to the data latch DL-L and the sense latch SL is clearedagain.

Referring to FIGS. 131 and 132, data are finally read at the thirddetermination level Vj3 and the results are stored in the sense latch SLin the third processing step of the read operation. The chip controlcircuit 200 sets the data of the data latch DL-R to “1” only on such abit position that both of the data stored in the sense latch SL and thedata in the data latch DL-R are “0”.

The data latches DL-L and DL-R sequentially output the data DQ4 to DQ7and DQ0 to DQ3 respectively.

Also in the read operation, the data are output after all three readoperations are defined.

For example, Japanese Patent Laying-Open No. 9-297996 (1997) discloses amore detailed structure of such a multivalued memory. This memory alsorequires a plurality of processing steps in each of read and writeoperations, similarly to the aforementioned multivalued memory.

In the conventional AND four-valued flash memory 8000, as hereinabovedescribed, the speed of the chip for read and write operations isdeteriorated by a delay following a plurality of processing steps, ascompared with the general two-valued flash memory. This problem ofdeterioration of the speed gets serious as the number of values isincreased.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a nonvolatilesemiconductor memory device capable of suppressing deterioration ofspeed also when holding multivalued data in a single memory cell.

Briefly stated, the present invention is directed to a nonvolatilesemiconductor memory device comprising a memory cell array, a cellselection circuit, a data read/write circuit and a data input/outputcircuit.

A plurality of memory cells are arranged in the memory cell array. Eachmemory cell includes a storage element holding binary data of n bits inresponse to 2^(n) (n: natural number) threshold levels.

The threshold levels correspond to levels obtained by rearranging a setof the binary data of n bits in a procedure corresponding to a procedureof:

i) associating n bit pointer variables BP(i) (i: natural number, 1≦i≦n)with n integers from zero to (n−1) arranged in arbitrary orderrespectively,

ii) rearranging the set of the binary data of n bits in two data groupsin response to whether data of a BP(1)-th bit is “0” or “1” in a firststep, and

ii) rearranging each group of the set of the binary data of n bitsdivided into 2^(j−1) groups in the process up to a (j−1)th step furtherin two data groups in response to whether data of a BP(j)-th bit is “0”or “1” in a j-th step (j: natural number, 2≦j≦n).

The cell selection circuit collectively selects a plurality of memorycells from the memory cell array in response to an address signal.

The data read/write circuit performs a read/write operation of storeddata on the selected plurality of memory cells on the basis of (2^(n)−1)determination levels corresponding to boundaries between groups of thethreshold levels corresponding to the data groups.

The data input/output circuit transfers the stored data between anexternal device for the nonvolatile semiconductor memory device and thememory cells as binary data through k input/output nodes every k bit (k:natural number).

The stored data held in each memory cell is generated from n bit datatransferred through the same input/output node at different timings.

Accordingly, a principal advantage of the present invention resides inthat multivalued data stored in a single memory cell is generated fromdata transferred at different timings and hence data can be output everytime each bit data is defined in a read operation, for reducing the dataoutput time.

Another advantage of the present invention resides in that multivalueddata stored in a single memory cell is generated from data transferredat different timings and hence data can be written every time each bitdata is defined in a write operation, for reducing the data input time.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing the structure of a flashmemory 1000 forming a nonvolatile semiconductor memory device accordingto a first embodiment of the present invention;

FIG. 2 illustrates data held in respective latches in a first processingstep of a read operation;

FIG. 3 illustrates thresholds of memory cells and determination levelsin the first processing step of the read operation;

FIG. 4 illustrates data held in the respective latches in a secondprocessing step of the read operation;

FIG. 5 illustrates thresholds of the memory cells and determinationlevels in the second processing step of the read operation;

FIG. 6 illustrates data held in the respective latches in a thirdprocessing step of the read operation;

FIG. 7 illustrates thresholds of the memory cells and determinationlevels in the third processing step of the read operation;

FIG. 8 is a timing chart for illustrating the read operation of theflash memory 1000 according to the first embodiment;

FIGS. 9 to 12 show first to fourth examples of association between writelevels and write data having upper and lower bits indefinable in singleread processing;

FIG. 13 illustrates data held in respective latches in a secondprocessing step of a read operation in a second modification of thefirst embodiment;

FIG. 14 illustrates thresholds of memory cells and determination levelsin the second processing step of the read operation in the secondmodification of the first embodiment;

FIG. 15 is a timing chart for illustrating the read operation of a flashmemory according to the second modification of the first embodiment;

FIGS. 16A and 16B illustrate the concept of a system of switchingconnection between a sense latch circuit 120 and bit lines in a flashmemory according to a second embodiment of the present invention;

FIG. 17 illustrates a circuit structure for implementing the conceptshown in FIGS. 16A and 16B;

FIG. 18 illustrates data held in respective latches in a firstprocessing step of a read operation;

FIG. 19 illustrates thresholds of memory cells and determination levelsin the first processing step of the read operation;

FIG. 20 illustrates data held in the respective latches in a secondprocessing step of the read operation;

FIG. 21 illustrates thresholds of the memory cells and determinationlevels in the second processing step of the read operation;

FIG. 22 illustrates data held in the respective latches in a thirdprocessing step of the read operation;

FIG. 23 illustrates thresholds of the memory cells and determinationlevels in the third processing step of the read operation;

FIG. 24 illustrates data held in the respective latches in a fourthprocessing step of the read operation;

FIG. 25 illustrates thresholds of the memory cells and determinationlevels in the fourth processing step of the read operation;

FIG. 26 illustrates data held in the respective latches in a fifthprocessing step of the read operation;

FIG. 27 illustrates thresholds of the memory cells and determinationlevels in the fifth processing step of the read operation;

FIG. 28 illustrates data held in the respective latches in a sixthprocessing step of the read operation;

FIG. 29 illustrates thresholds of the memory cells and determinationlevels in the sixth processing step of the read operation;

FIG. 30 is a timing chart for illustrating a read operation of the flashmemory according to the second embodiment of the present invention;

FIG. 31 illustrates data held in respective latches in a firstprocessing step of a read operation in a flash memory according to athird embodiment of the present invention;

FIG. 32 illustrates thresholds of memory cells and determination levelsin the first processing step of the read operation;

FIG. 33 illustrates data held in the respective latches in a secondprocessing step of the read operation;

FIG. 34 illustrates thresholds of the memory cells and determinationlevels in the second processing step of the read operation;

FIG. 35 illustrates data held in the respective latches in a thirdprocessing step of the read operation;

FIG. 36 illustrates thresholds of the memory cells and determinationlevels in the third processing step of the read operation;

FIG. 37 is a timing chart for illustrating a write operation of theflash memory according to the third embodiment;

FIG. 38 illustrates data held in the respective latches in a secondprocessing step of the write operation;

FIG. 39 illustrates thresholds of memory cells and determination levelsin a read operation in the second processing step of the writeoperation;

FIG. 40 illustrates data held in respective latches in a secondprocessing step of a write operation in a flash memory according to afourth embodiment of the present invention;

FIG. 41 illustrates thresholds of memory cells and determination levelsin the second processing of the write operation;

FIG. 42 illustrates association between write data writable in theprocedure of the first embodiment and threshold levels of the memorycell transistors;

FIG. 43 illustrates association between write data unwritable in theprocedure of the first embodiment and threshold levels of the memorycell transistors;

FIG. 44 illustrates data held in respective latches in a firstprocessing step of a read operation in a flash memory according to afifth embodiment of the present invention;

FIG. 45 illustrates thresholds of memory cells and determination levelsin the first processing step of the read operation in the fifthembodiment;

FIG. 46 illustrates data held in the respective latches in a secondprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 47 illustrates thresholds of the memory cells and determinationlevels in the second processing step of the read operation in the fifthembodiment;

FIG. 48 illustrates data held in the respective latches in a thirdprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 49 illustrates thresholds of the memory cells and determinationlevels in the third processing step of the read operation in the fifthembodiment;

FIG. 50 illustrates data held in the respective latches in a fourthprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 51 illustrates thresholds of the memory cells and determinationlevels in the fourth processing step of the read operation in the fifthembodiment;

FIG. 52 illustrates data held in the respective latches in a fifthprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 53 illustrates thresholds of the memory cells and determinationlevels in the fifth processing step of the read operation in the fifthembodiment;

FIG. 54 illustrates data held in the respective latches in a sixthprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 55 illustrates thresholds of the memory cells and determinationlevels in the sixth processing step of the read operation in the fifthembodiment;

FIG. 56 illustrates data held in the respective latches in a seventhprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 57 illustrates thresholds of the memory cells and determinationlevels in the seventh processing step of the read operation in the fifthembodiment;

FIG. 58 illustrates data held in the respective latches in an eighthprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 59 illustrates thresholds of the memory cells and determinationlevels in the eighth processing step of the read operation in the fifthembodiment;

FIG. 60 illustrates data held in the respective latches in a ninthprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 61 illustrates thresholds of the memory cells and determinationlevels in the ninth processing step of the read operation in the fifthembodiment;

FIG. 62 illustrates data held in the respective latches in a tenthprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 63 illustrates thresholds of the memory cells and determinationlevels in the tenth processing step of the read operation in the fifthembodiment;

FIG. 64 illustrates data held in the respective latches in an eleventhprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 65 illustrates thresholds of the memory cells and determinationlevels in the eleventh processing step of the read operation in thefifth embodiment;

FIG. 66 illustrates data held in the respective latches in a twelfthprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 67 illustrates thresholds of the memory cells and determinationlevels in the twelfth processing step of the read operation in the fifthembodiment;

FIG. 68 illustrates data held in the respective latches in a thirteenthprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 69 illustrates thresholds of the memory cells and determinationlevels in the thirteenth processing step of the read operation in thefifth embodiment;

FIG. 70 illustrates data held in the respective latches in a fourteenthprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 71 illustrates thresholds of the memory cells and determinationlevels in the fourteenth processing step of the read operation in thefifth embodiment;

FIG. 72 illustrates data held in the respective latches in a fifteenthprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 73 illustrates thresholds of the memory cells and determinationlevels in the fifteenth processing step of the read operation in thefifth embodiment;

FIG. 74 illustrates data held in the respective latches in a sixteenthprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 75 illustrates thresholds of the memory cells and determinationlevels in the sixteenth processing step of the read operation in thefifth embodiment;

FIG. 76 illustrates data held in the respective latches in a seventeenthprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 77 illustrates thresholds of the memory cells and determinationlevels in the seventeenth processing step of the read operation in thefifth embodiment;

FIG. 78 illustrates data held in the respective latches in an eighteenthprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 79 illustrates thresholds of the memory cells and determinationlevels in the eighteenth processing step of the read operation in thefifth embodiment;

FIG. 80 illustrates data held in the respective latches in a nineteenthprocessing step of the read operation in the fifth embodiment of thepresent invention;

FIG. 81 illustrates thresholds of the memory cells and determinationlevels in the nineteenth processing step of the read operation in thefifth embodiment;

FIG. 82 illustrates data held in the respective latches in a firstprocessing step of a write operation in the fifth embodiment of thepresent invention;

FIG. 83 illustrates thresholds of the memory cells and determinationlevels in the first processing step of the write operation in the fifthembodiment;

FIG. 84 illustrates data held in the respective latches in second andthird processing steps of the write operation in the fifth embodiment ofthe present invention;

FIG. 85 illustrates thresholds of the memory cells and determinationlevels in the second and third processing steps of the write operationin the fifth embodiment;

FIG. 86 illustrates data held in the respective latches in fourth toseventh processing steps of the write operation in the fifth embodimentof the present invention;

FIG. 87 illustrates thresholds of the memory cells and determinationlevels in the fourth to seventh processing steps of the write operationin the fifth embodiment;

FIG. 88 illustrates data held in respective latches in a firstprocessing step of a write operation in a flash memory according to asixth embodiment of the present invention;

FIG. 89 illustrates thresholds of memory cells and determination levelsin the first processing step of the write operation in the sixthembodiment;

FIG. 90 illustrates data held in the respective latches in a secondprocessing step of the write operation in the sixth embodiment of thepresent invention;

FIG. 91 illustrates thresholds of the memory cells and determinationlevels in the second processing step of the write operation in the sixthembodiment;

FIG. 92 illustrates data held in the respective latches in a thirdprocessing step of the write operation in the sixth embodiment of thepresent invention;

FIG. 93 illustrates thresholds of the memory cells and determinationlevels in the third processing step of the write operation in the sixthembodiment;

FIG. 94 illustrates data held in the respective latches in a fourthprocessing step of the write operation in the sixth embodiment of thepresent invention;

FIG. 95 illustrates thresholds of the memory cells and determinationlevels in the fourth processing step of the write operation in the sixthembodiment;

FIG. 96 illustrates data held in the respective latches in a fifthprocessing step of the write operation in the sixth embodiment of thepresent invention;

FIG. 97 illustrates thresholds of the memory cells and determinationlevels in the fifth processing step of the write operation in the sixthembodiment;

FIG. 98 illustrates data held in the respective latches in a sixthprocessing step of the write operation in the sixth embodiment of thepresent invention;

FIG. 99 illustrates thresholds of the memory cells and determinationlevels in the sixth processing step of the write operation in the sixthembodiment;

FIG. 100 illustrates data held in the respective latches in a seventhprocessing step of the write operation in the sixth embodiment of thepresent invention;

FIG. 101 illustrates thresholds of the memory cells and determinationlevels in the seventh processing step of the write operation in thesixth embodiment;

FIG. 102 illustrates data held in the respective latches in an eighthprocessing step of the write operation in the sixth embodiment of thepresent invention;

FIG. 103 illustrates thresholds of the memory cells and determinationlevels in the eighth processing step of the write operation in the sixthembodiment;

FIG. 104 illustrates data held in the respective latches in a ninthprocessing step of the write operation in the sixth embodiment of thepresent invention;

FIG. 105 illustrates thresholds of the memory cells and determinationlevels in the ninth processing step of the write operation in the sixthembodiment;

FIG. 106 illustrates data held in the respective latches in a tenthprocessing step of the write operation in the sixth embodiment of thepresent invention;

FIG. 107 illustrates thresholds of the memory cells and determinationlevels in the tenth processing step of the write operation in the sixthembodiment;

FIG. 108 illustrates data held in the respective latches in an eleventhprocessing step of the write operation in the sixth embodiment of thepresent invention;

FIG. 109 illustrates thresholds of the memory cells and determinationlevels in the eleventh processing step of the write operation in thesixth embodiment;

FIG. 110 illustrates data held in the respective latches in a twelfthprocessing step of the write operation in the sixth embodiment of thepresent invention;

FIG. 111 illustrates thresholds of the memory cells and determinationlevels in the twelfth processing step of the write operation in thesixth embodiment;

FIG. 112 illustrates data held in the respective latches in a thirteenthprocessing step of the write operation in the sixth embodiment of thepresent invention;

FIG. 113 illustrates thresholds of the memory cells and determinationlevels in the thirteenth processing step of the write operation in thesixth embodiment;

FIG. 114 illustrates data held in the respective latches in a fourteenthprocessing step of the write operation in the sixth embodiment of thepresent invention;

FIG. 115 illustrates thresholds of the memory cells and determinationlevels in the fourteenth processing step of the write operation in thesixth embodiment;

FIG. 116 illustrates data held in the respective latches in a fifteenthprocessing step of the write operation in the sixth embodiment of thepresent invention;

FIG. 117 illustrates thresholds of the memory cells and determinationlevels in the fifteenth processing step of the write operation in thesixth embodiment;

FIG. 118 is a schematic block diagram showing the overall structure of aconventional AND flash memory 8000;

FIG. 119 illustrates the relation between write data and thresholds ofmemory cell transistors in a conventional two-valued AND flash memory;

FIG. 120 illustrates the relation between write data and thresholds ofmemory cell transistors in the conventional four-valued AND flash memory8000;

FIG. 121 illustrates data held in respective latches in a firstprocessing step of a conventional write operation;

FIG. 122 illustrates thresholds of memory cells in the first processingstep of the conventional write operation;

FIG. 123 illustrates data held in the respective latches in a secondprocessing step of the conventional write operation;

FIG. 124 illustrates thresholds of the memory cells in the secondprocessing step of the conventional write operation;

FIG. 125 illustrates data held in the respective latches in a thirdprocessing step of the conventional write operation;

FIG. 126 illustrates thresholds of the memory cells in the thirdprocessing step of the conventional write operation;

FIG. 127 illustrates data held in the respective latches in a firstprocessing step of a conventional read operation;

FIG. 128 illustrates thresholds of memory cells in the first processingstep of the conventional read operation;

FIG. 129 illustrates data held in the respective latches in a secondprocessing step of the conventional read operation;

FIG. 130 illustrates thresholds of the memory cells in the secondprocessing step of the conventional read operation;

FIG. 131 illustrates data held in the respective latches in a thirdprocessing step of the conventional read operation; and

FIG. 132 illustrates thresholds of the memory cells in the thirdprocessing step of the conventional read operation;

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a schematic block diagram showing the structure of a flashmemory 1000 forming a nonvolatile semiconductor memory device accordingto a first embodiment of the present invention.

A memory cell array 100 has a number of memory cells including floatinggates and control gates. Also in FIG. 1, the memory cell array 100 isdivided into two memory cell blocks 100R and 100L.

The control gates, drains and sources of the memory cells are connectedto word lines WL, bit lines BL and source lines SCL (not shown)respectively.

FIG. 1 representatively shows two word lines WL and two bit lines BLrespectively.

Row decoders 110 selectively drive the word lines WL on the basis ofaddress signals A0 to Ak externally supplied through an address signalinput terminal group 12 and an address buffer 146. A sense latch circuit120 is provided on single ends of the bit lines BL. The bit lines BL areselected on the basis of selection signals output from column decoders130 in response to the address signals A0 to Ak, so that read data andwrite data are transmitted to/received in the selected bit lines BL.

The sense latch circuit 120 includes a column switching circuit (notshown) for selecting the bit lines BL on the basis of the selectionsignals from the column decoders 130.

A chip control circuit 200 includes a control circuit 210 receiving anaccess control signal and a clock signal externally supplied from acontrol signal input terminal group 14 through a command buffer circuit144 and totally controlling the internal circuits of the flash memory1000 for write control and read control of the memory cells. The controlcircuit 210 controls a power supply generation part 150 therebyswitching operating voltages of word drivers (not shown) driving thepotentials of the word lines WL in response to operation modes forerasing, writing, reading and the like.

Data latch circuits DL-L and DL-R are data buffers temporarily holdingdata transferred in data write and read operations.

The chip control part 200 further includes a read data conversioncircuit 220 controlled by the control circuit 210 for operating dataheld in the data latch circuits DL-L and DL-R and the sense latchcircuit 120 and generating read data in a read operation and a writedata conversion circuit 230 controlled by the control circuit 210 foroperating data held in the data latch circuits DL-L and DL-R and thesense latch circuit 120 and generating write data in a write operation.

The operation modes of the flash memory 1000 are not particularlyrestricted but instructed by the access control signal externallysupplied to the chip control part 200 or command data supplied through adata bus or the like, and include data rewrite (erase and write) anddata read modes.

Also in the AND four-valued flash memory 1000 shown in FIG. 1, eachmemory cell is in an information storage state selected from an erasedstate, a first write state, a second write state and a third writestate.

Therefore, the flash memory 1000 also sets three different types ofwrite verify voltages applied to the word lines WL in the writeoperation and performs the write operation three times whilesequentially switching the write verify voltages. The flash memory 1000enables reduction of write and read times by devising arrangement of2-bit data corresponding to the aforementioned four information storagestates and the order of the write processing performed three times.

In each write operation, the control circuit 210 controls the write dataconversion circuit 230 to operate two-valued (1-bit) write data “0” or“1” (“L” or “H”) to be held in a sense latch SL (included in the senselatch circuit 120) in each write operation on the basis of write dataheld in the data latches DL-L and DL-R. The control circuit 210 sets thewrite verify voltage corresponding to each of the three write operationsfor controlling the write operation.

In the structure of the conventional flash memory 8000, the data latchcircuits DL-L and DL-R hold two data strings obtained by dividing thedata string supplied to the data input/output terminal group 10 at thesame timing respectively.

In the flash memory 1000 according to this embodiment, the data latchcircuits DL-L and DL-R hold two data strings supplied to the datainput/output terminal group 10 at different timings and supplied to thesame memory cell group coupled to the same word line WL or two datastrings supplied to memory cell groups coupled to the same word line WLand different bit lines BL respectively dissimilarly to the conventionalflash memory 8000, as described later. Information of four values (twobits) can be written in a single memory cell due to this structure.

The flash memory 1000 also sets three types of voltages as word lineselection levels applied by the word lines WL in the read operation andcaptures two-valued (1-bit) data read from the memory cells in threeread operations in the data latch circuits DL-L and DL-R through thesense latch circuit 120. During the three read operations, the read dataconversion circuit 220 sequentially converts the data held in the latchcircuits DL-L and DL-R to information of four values (two bits) and thecontrol circuit 210 outputs the information from the data input/outputterminal group 10.

The aforementioned write and read operations are now described infurther detail.

[Read Operation of Four-Valued Data]

In the flash memory 1000 according to the first embodiment, the methodof storing input/output data in the data latches DL-L and DL-R isdifferent from that in the conventional flash memory 8000, ashereinabove described.

In the conventional multivalued flash memory 8000, a single memory cellstores data formed by those in the same byte such as the data DQ0 andDQ4 or DQ1 and DQ5 in 1-byte data input from the data input/outputterminal group 10 at the same timing. In the four-valued flash memory1000 according to the present invention, a single memory cell storesdata having the same Y addresses of different word lines andcorresponding to bits of the same I/O data or data corresponding to thesame I/O data (data corresponding to the same I/O data on different Yaddresses in the same sector) having somewhat different Y addresses inthe same sector (the same word line WL).

FIGS. 2 to 7, illustrating operations of the flash memory 1000 accordingto the first embodiment, are conceptual diagrams showing data held inthe data latches DL-L and DL-R and the sense latch SL and thresholds ofmemory cells and determination levels in reading in first to thirdprocessing steps of the read operation.

FIG. 2 shows data held in the respective latches DL-L, DL-R and SL inthe first processing step of the read operation, and FIG. 3 showsthresholds of the memory cells and determination levels in the firstprocessing step of the read operation.

FIG. 4 shows data held in the respective latches DL-L, DL-R and SL inthe second processing step of the read operation, and FIG. 5 showsthresholds of the memory cells and determination levels in the secondprocessing step of the read operation.

FIG. 6 shows data held in the respective latches DL-L, DL-R and SL inthe third processing step of the read operation, and FIG. 7 showsthresholds of the memory cells and determination levels in the thirdprocessing step of the read operation.

Referring to FIGS. 2 and 3, data are collectively read from a pluralityof memory cells for 1 KB, for example, connected to the same word lineat a second determination level Vj2 and the results are stored in thesense latch circuit 120 in the first processing step of the readoperation.

FIG. 2 extractively shows the sense latch SL, included in the senselatch circuit 120, holding data for one byte first output from the datainput/output terminal group 10 in a read operation for one sector anddata latches DL-L and DL-R corresponding thereto. Processing similar tothat described below is performed in parallel on data read from the datainput/output terminal group 10 subsequently to the data for one byte.

It is assumed that memory cells MC0 to MC7 holding a data stringincluding the data for one byte first output from the data input/outputterminal group 10 hold data “11”, “01”, “00”, “10”, “01”, “00”, “10” and“11” respectively. Referring to FIG. 2, therefore, the sense latchcircuit 120 holds data C9 h in hexadecimal notation in an area for onebyte.

The data collectively read at the second determination level Vj2 arestored in the data latch DL-L from the sense latch SL. At this time, thedata stored in the data latch DL-L are sequentially output byte by bytefrom the data input/output terminal group 10 under control of thecontrol circuit 210. The data stored in the data latch circuit DL-L atthis point of time correspond to half data for one sector in a readoperation of the conventional two-valued flash memory.

Thus, the read operation is first performed at the second determinationlevel Vj2 in the first processing step so that four types of data in thememory cells are defined as “0” or “1” in reading at this level, asshown in FIG. 3. In other words, it follows that “0” or “1” of the upperbits of the data stored in the memory cells MC0 to MC7 are defined.

Referring to FIGS. 4 and 5, data are read at a first determination levelVj1 in the second processing step of the read operation when the dataare stored in the data latch DL-L from the sense latch SL and the senselatch SL is cleared in the first processing step, and the read data arestored in the sense latch circuit 120. In other words, data aretransferred from the sense latch SL to the data latch DL-R while thedata latch circuit DL-L performs data output. After data transfer to thedata latch DL-R, the sense latch circuit 120 is cleared.

Referring to FIGS. 6 and 7, data are read at a third determination levelVj3 and stored in the sense latch circuit 120 in the third processingstep of the read operation. The read data conversion circuit 220performs an operation between the data held in the sense latch SL andthe data held in the data latch DL-R, and changes bit data of the datalatch DL-R only where both bit data are “0”.

After data output from the data latch DL-L is terminated, the controlcircuit 210 starts data output from the data latch DL-R to the datainput/output terminal group 10.

The conventional four-valued flash memory 8000 has a sector size of 2KB, for example, and performs an output operation at 50 nm per byte.Therefore, the conventional flash memory 8000 requires a time of 50 μsfor outputting data of 1 KB, i.e., half the sector size.

Also in the conventional four-valued flash memory 8000, the time(referred to as a first access time) up to immediately before output iscompleted in 50 μs also when starting data output after performing theread operation three times. Therefore, it is possible to complete theaccess operations from the aforementioned first to third determinationlevels while outputting data for half a sector in the four-valued flashmemory 1000 according to the present invention.

In the four-valued flash memory 1000, therefore, a time for obtainingthe result of determination at the second determination level Vj2 mayelapse before data output after input of a read command. This isequivalent to a read time for two values, and hence no speeddeterioration is caused by the four-valued data stored in the memorycells.

If the output speed is extremely high (or the first access time isextremely long) and output from the data latch DL-L is terminated beforethe data in the data latch DL-R are defined, it follows that a latencyoccurs between the data output from the data latch DL-L and that fromthe data latch DL-R. Also in this case, the read speed is improved bythe output time of the data latch DL-L as compared with the conventionaldata read operation.

In general, however, the sector size is adjusted to cause no suchlatency. In other words, the sector size (the quantity of data outputfrom memory cells connected with a single word line) may be decided inresponse to the read time (first access time).

In order to satisfy this condition, the sector size must satisfy thefollowing expression:{sector size (byte)×(output time per byte)×½}≧{single read time×2}  (1)

FIG. 8 is a timing chart for illustrating the read operation of theflash memory 1000 according to the first embodiment.

Referring to FIG. 8, data are first read at the second determinationlevel Vj2 and stored in the sense latch circuit 120 at a time t1.

Then, the data held in the sense latch circuit 120 are transferred tothe data latch DL-L and the sense latch circuit 120 is cleared at a timet2.

At a time t3, a read operation based on the first determination levelVj1 is started. At a time t4, output of data for a half sector held inthe data latch DL-L is started under control of the control circuit 210.

At a time t5, the data read on the basis of the first determinationlevel Vj1 and held in the sense latch circuit 120 are stored in the datalatch DL-R. On the other hand, the sense latch circuit 120 is cleared.

At a time t6, a read operation based on the third determination levelVj3 is started and read data are stored in the sense latch circuit 120.

After completion of the read operation based on the third determinationlevel Vj3, the read data conversion circuit 220 performs an operationbetween the data held in the sense latch circuit 120 and the data heldin the data latch circuit DL-R under control of the control circuit 210,and changes bit data held in the data latch DL-R to “1” only where bothbit data are “0”.

At a time t9, output of data for the remaining half sector held in thedata latch DL-R is started. At a time t10, data output for one sector iscompleted.

While the data string read at the time t1 is output, the read operationof the data string to be subsequently output is completed as hereinabovedescribed. Thus, a delay time can be reduced when outputting read datafrom four-valued memory cells.

[First Modification of First Embodiment]

In the first embodiment, the processing shown in FIGS. 4 and 5 and thatshown in FIGS. 6 and 7 are performed while outputting the data held inthe data latch DL-L after performing the processing shown in FIGS. 2 and3. However, the present invention is not necessarily restricted to thisorder of processing but the processing shown in FIGS. 2 and 3 may beperformed after performing the processing shown in FIGS. 4 and 5 andthat shown in FIGS. 6 and 7, for example. In other words, the processingshown in FIGS. 2 and 3 may be performed while outputting data from thedata latch DL-R after reading data at the first determination level Vj1,storing the result of reading in the data latch DL-R, then reading dataat the third determination level Vj3 and performing an operation betweenthe data stored in the sense latch SL and the data held in the datalatch DL-R. In this case, it follows that output of the data read at thesecond determination level and stored in the data latch DL-L is startedafter data output from the data latch DL-R is terminated.

In this case, data reading can be performed in a shorter time than theconventional four-valued flash memory 8000 although the effect ofreducing the read operation time is smaller than that in the firstembodiment.

The first to third processing steps may not necessarily be continuouslyperformed as described above but the data held in the memory cells MC0to MC7 may be read bitwise from the upper or lower bits.

[Second Modification of First Embodiment]

In the first embodiment, as hereinabove described, the write levels andthe write data are so associated that four types of data in the memorycells are defined as “0” or “1” by first performing the read operationat the second determination level Vj2 in the first processing step ofthe read operation.

FIGS. 9 to 12 illustrate exemplary association of write levels and writedata capable of defining neither upper bits nor lower bits in singleread processing. Referring to each of FIGS. 9 to 12, neither upper bitsnor lower bits can be defined in single read processing also when “0”and “1” are replaced with each other.

A read operation according to a second modification of the firstembodiment provides another exemplary association of write data andwrite levels capable of reducing the read time similarly to the firstembodiment.

FIG. 13 illustrates data held in the respective latches in a secondprocessing step of the read operation according to the secondmodification of the first embodiment, and FIG. 14 illustrates thresholdsof the memory cells and determination levels in the second processingstep of the read operation according to the second modification of thefirst embodiment.

Also when the data “00” and “01” are replaced with each other in thedata arrangement shown in FIG. 3 as shown in FIG. 14, the read time canbe reduced by processing similar to that of the first embodiment.

In this case, data are collectively read from a plurality of memorycells connected to the same word line at the second determination levelVj2 and the results are stored in the sense latch circuit 120 in a firstprocessing step of the read operation.

It is assumed that the memory cells MC0 to MC7 holding a data stringincluding data for one byte first output from the data input/outputterminal group 10 hold data “11”, “00”, “01”, “10”, “00”, “01”, “10” and“11” respectively. Therefore, the data latch circuit DL-L holds data C9h in hexadecimal notation in an area for one byte.

The data collectively read at the second determination level Vj2 arestored in both the data latches DL-L and DL-R from the sense latch SL.At this point of time, the data stored in the data latch DL-L aresequentially output one by one from the data input/output terminal group10 under control of the control circuit 210. The data stored in the datalatch circuits DL-L and DL-R at this point of time correspond to halfthe data for one sector in the read operation of the conventionaltwo-valued flash memory.

Also in the first processing step of the read operation according to thesecond modification of the first embodiment, upper bits of four types ofdata in the memory cells are defined as “0” or “1” by first performingthe read operation at the second determination level Vj2.

Referring to FIGS. 13 and 14, data are read at the first determinationlevel Vj1 in the second processing step of the read operation when thedata are stored in the data latches DL-L and DL-R from the sense latchSL and the sense latch SL is cleared in the first processing step, andthe read data are stored in the sense latch circuit 120. The read dataconversion circuit 220 performs an operation between the data held inthe sense latch SL and the data held in the data latch DL-R while thedata are output from the data latch DL-L and zeros corresponding bitdata of the data latch DL-R when bit data of the sense latch SL are “0”and the corresponding bit data of the data latch DL-R are “1” whileotherwise changing the bit data of the data latch DL-R to “1”. Afterdata change in the data latch DL-R, the sense latch circuit 120 iscleared.

In a third processing step of the read operation, data are read at thethird determination level Vj3 and stored in the sense latch circuit 120.The read data conversion circuit 220 performs an operation between thedata held in the sense latch SL and the data held in the data latch DL-Rand changes bit data of the data latch DL-R only where both bit data are“0”.

After data output from the data latch DL-L is terminated, the controlcircuit 210 starts data output from the data latch DL-R to the datainput/output terminal group 10.

FIG. 15 is a timing chart for illustrating the read operation of theflash memory according to the second modification of the firstembodiment.

Referring to FIG. 15, data are read at the second determination levelVj2 and stored in the sense latch circuit 120 at a time t1.

At a time t2, the data held in the sense latch circuit 120 aretransferred to the data latches DL-L and DL-R, and the sense latchcircuit 120 is cleared.

At a time t3, a read operation based on the first determination levelVj1 is started. At a time t4, output of data for a half sector held inthe data latch DL-L is started under control of the control circuit 210.

The read data conversion circuit 220 starts an operation between dataheld in the sense latch SL and data held in the data latch DL-R at atime t5, and zeros corresponding bit data of the data latch DL-R whenbit data of the sense latch SL are “0” and the corresponding bit data ofthe data latch DL-R are “1” while otherwise changing the bit data of thedata latch DL-R to “ ”.

At a time t7, the sense latch circuit 120 is cleared.

At a time t8, a read operation based on the third determination levelVj3 is started and read data are stored in the sense latch circuit 120.

After completion of the read operation based on the third determinationlevel Vj3, the read data conversion circuit 220 starts an operationbetween the data held in the sense latch circuit 120 and the data heldin the data latch circuit DL-R under control of the control circuit 210at a time t9, and changes bit data held in the data latch DL-R to “1”only where both bit data are “0”.

At a time t11, output of data for the remaining half sector held in thedata latch circuit Dl-R is started.

At a time t12, data output for one sector is completed.

An effect similar to that of the read operation in the first embodimentis attained also by the aforementioned read operation.

Second Embodiment

FIGS. 16A and 16B illustrate the concept of a system of switchingconnection between a sense latch circuit 120 and bit lines BL1 and BL2of a flash memory according to a second embodiment of the presentinvention. Referring to FIG. 16A, the bit line BL1 is connected to asense latch SL corresponding to 1-bit data in the sense latch circuit120. Referring to FIG. 16B, the bit line BL2 is connected to theaforementioned sense latch SL.

The flash memory according to the second embodiment is basically similarin structure to the flash memory 1000 according to the first embodimentexcept the structure shown in FIGS. 16A and 16B and a control operationof a control circuit 210.

When the sense latch circuit 120 captures data from the bit lines in adivided manner as to adjacent memory cells while a single word line WLis selected, first to third processing steps can be dividedly performedin parallel with each other every data reading from the memory cells dueto the structure shown in FIGS. 16A and 16B, for attaining further speedincrease as described below.

FIG. 17 illustrates a circuit structure for implementing the conceptshown in FIGS. 16A and 16B. This figures extractively shows arepresentative structure related to two of sense latches correspondingto respective bit data in the sense latch circuit 120.

Referring to FIG. 17, drains of memory cell transistors MC1 nm, MC2 nm,MC1 nm+1 and Mc2 nm+1 having gates connected with a single word line WLnare connected with sub bit lines SBL1 m, SBL2 m, SBL1 m+1 and SBL2 m+1respectively, for example. The memory cell transistors MC1 nm, MC2 nm,MC1 nm+1 and Mc2 nm+1 are the so-called floating gate transistors havingcontrol gates and floating gates.

The sub bit lines SBL1 m and SBL2 m are connected with a main bit lineMBLm through transistors Tr1 m and Tr2 m respectively. Gates of thetransistors Tr1 m and Tr2 m are controlled by signals BSS1 and BSS2respectively.

The sub bit lines SBL1 m+1 and SBL2 m+1 are connected with a main bitline MBLm+1 through transistors Tr1 m+1 and Tr2 m+1 respectively. Gatesof the transistors Tr1 m+1 and Tr2 m+1 are controlled by the signalsBSS1 and BSS2 respectively.

The main bit line MBLm is connected with a latch circuit SLmcorresponding to 1-bit data in the sense latch circuit 120 through agate transistor TGm having a gate potential controlled by a signal STG.The main bit line MBLm+1 is connected with a latch circuit SLm+1corresponding to another 1-bit data in the sense latch circuit 120through a gate transistor TGm+1 having a gate potential controlled bythe signal STG.

The number of sub bit lines connected to each main bit line is notrestricted to two as shown in FIG. 17 but a larger number is employable.

FIGS. 18 to 29, illustrating operations of the flash memory according tothe second embodiment, are conceptual diagrams showing data held in datalatches DL-L and DL-R and a sense latch SL as well as thresholds ofmemory cells and determination levels in reading in first to sixthprocessing steps of a read operation.

FIG. 18 shows data held in the respective latches in the firstprocessing step of the read operation, and FIG. 19 shows thresholds ofthe memory cells and determination levels in the first processing stepof the read operation.

FIG. 20 shows data held in the respective latches in the secondprocessing step of the read operation, and FIG. 21 shows thresholds ofthe memory cells and determination levels in the second processing stepof the read operation.

FIG. 22 shows data held in the respective latches in the thirdprocessing step of the read operation, and FIG. 23 shows thresholds ofthe memory cells and determination levels in the third processing stepof the read operation.

FIG. 24 shows data held in the respective latches in the fourthprocessing step of the read operation, and FIG. 25 shows thresholds ofthe memory cells and determination levels in the fourth processing stepof the read operation.

FIG. 26 shows data held in the respective latches in the fifthprocessing step of the read operation, and FIG. 27 shows thresholds ofthe memory cells and determination levels in the fifth processing stepof the read operation.

FIG. 28 shows data held in the respective latches in the sixthprocessing step of the read operation, and FIG. 29 shows thresholds ofthe memory cells and determination levels in the sixth processing stepof the read operation.

Referring to FIGS. 18 and 19, data are collectively read from aplurality of memory cells for 1 KB, for example, connected to the sameword line at a second determination level Vj2 and the results are storedin the sense latch circuit 120 in the first processing step of the readoperation.

FIG. 18 extractively shows a sense latch SL, included in the sense latchcircuit 120, holding data for 1 byte first output from a datainput/output terminal group 10 in a read operation for one sector anddata latches DL-L and DL-R corresponding thereto. Processing similar tothat described below is performed in parallel on data read from the datainput/output terminal group 10 subsequently to the data for one byte.

It is assumed that memory cells MC1 n 0 to MC1 n 7 connected to the subbit lines SBL1 m and SBL1 m+1 selected by the signal BSS1 among the subbit lines shown in FIG. 17 for holding a data string including the datafor one byte first output from the data input/output terminal group 10hold data “11”, “01”, “00”, “10”, “01”, “00”, “10” and “11 respectively.Referring to FIG. 18, therefore, the sense latch circuit 120 holds dataC9 h in hexadecimal notation in an area for one byte.

The data collectively read at the second determination level Vj2 arestored in the data latch DL-L from the sense latch SL. At this time, thedata stored in the data latch DL-L are sequentially output byte by bytefrom the data input/output terminal group 10 under control of a controlcircuit 210. The data stored in the data latch DL-L at this point oftime correspond to ¼ those for one sector in the read operation of theconventional two-valued flash memory.

Thus, the read operation is first performed at the second determinationlevel Vj2 in the first processing step of the read operation so thatfour types of data in the memory cells are defined as “0” or “1” asshown in FIG. 19. In other words, it follows that “0” or “1” of theupper bits of the data stored in the memory cells MC1 n 0 to MC1 n 7 aredefined.

Referring to FIGS. 20 and 21, data are read from memory cells MC2 n 0 toMC2 n 7 connected with the sub bit lines SBL2 m and SBL2 m+1 selected bythe signal BSS2 among the sub bit lines shown in FIG. 17 at the seconddetermination level Vj2 in the second processing step of the readoperation when the data are stored in the data latch DL-L from the senselatch SL in the first processing step, and the read data are stored inthe sense latch circuit 120.

It is assumed that the memory cells MC2 n 0 to MC2 n 7 also hold data“11”, “01”, “00”, “10”, “01”, “00”, “10” and “11” respectively.Referring to FIG. 20, therefore, the sense latch circuit 120 also holdsdata C9 h in hexadecimal notation in an area for one byte.

In other words, the data are transferred from the sense latch SL to thedata latch DL-R while data output is performed from the data latchcircuit DL-L. Following data output from the data latch DL-L, the dataare output from the data latch DL-R.

Similar processing is performed in parallel also as to memory cells,other than the memory cells MC2 n 0 to MC2 n 7, connected to the sameword line and sub bit lines selected by the signal BSS2 among the subbit lines shown in FIG. 17.

Referring to FIGS. 22 and 23, data are read at a first determinationlevel Vj1 in the third processing step of the read operation when thedata are stored in the data latch DL-L from the sense latch SL and thesense latch SL is cleared in the second processing step, and the readdata are stored in the sense latch circuit 120.

Then, data are transferred from the sense latch SL to the data latchDL-L while data output is performed from the data latch circuit DL-R.After data transfer to the data latch DL-L, the sense latch circuit 120is cleared.

Referring to FIGS. 24 and 25, data are read from the memory cells MC1 n0 to MCC1 n 7 at a third determination level Vj3 and stored in the senselatch circuit 120 in the fourth processing step of the read operation.

A read data conversion circuit 220 performs an operation between thedata held in the sense latch circuit SL and the data held in the datalatch DL-L and changes bit data of the data latch DL-L to “1” only whereboth bit data are “0”. At this time, the data latch DL-R is in theprocess of outputting data.

After data output from the data latch DL-R is terminated, the controlcircuit 210 starts data output from the data latch DL-L to the datainput/output terminal group 10.

Referring to FIGS. 26 and 27, data are read from the memory cells MC2 n0 to MC2 n 0 at the first determination level Vj1 in the fifthprocessing step of the read operation when the data are stored in thedata latch DL-L from the sense latch SL and the sense latch SL iscleared in the fourth processing step, and the read data are stored inthe sense latch circuit 120.

Then, data are transferred from the sense latch SL to the data latchDL-R in the process of data output from the data latch DL-L. After datatransfer to the data latch DL-R, the sense latch circuit 120 is cleared.

Referring to FIGS. 28 and 29, data are read from the memory cells MC2 n0 to MC2 n 7 at the third determination level Vj3 and stored in thesense latch circuit 120 in the sixth processing step of the readoperation.

The read data conversion circuit 220 performs an operation between thedata held in the sense latch SL and the data held in the data latchDL-R, and changes bit data of the data latch DL-R to “1” only where bothbit data are “0”. At this time, the data latch DL-L is in the process ofdata output.

After data output from the data latch DL-L is terminated, the controlcircuit 210 starts data output from the data latch DL-R to the datainput/output terminal group 10.

In the four-valued flash memory according to the second embodiment,therefore, a time for obtaining the result of determination at thesecond determination level Vj2 for data for a quarter sector may elapsebefore data output after input of a read command. Thus, the read timecan be further reduced as compared with the first embodiment.

FIG. 30 is a timing chart for illustrating the read operation of theflash memory according to the second embodiment.

Referring to FIG. 30, data are first read from the memory cells MC1 n 0to MC1 n 7 etc. at the second determination level Vj2 and stored in thesense latch circuit 120 at a time t1.

At a time t2, the data held in the sense latch circuit 120 aretransferred to the data latch DL-L, and the sense latch circuit 120 iscleared.

At a time t3, a read operation from the memory cells MC2 n 0 to MC2 n 7etc. is started at the second determination level Vj2 and the read dataare stored in the sense latch circuit 120.

At a time t4, data output from the data latch DL-L is started.

At a time t5, data read from the memory cells MC2 n 0 to MC2 n 7 etc. atthe second determination level Vj2 are stored in the data latch DL-R.

At a time t6, a read operation from the memory cells MC1 n 0 to MC1 n 7etc. is started on the basis of the first determination level Vj1. At atime t7, output of read data from the memory cells MC2 n 0 to MC2 n 7etc. for a quarter sector held in the data latch DL-R is started undercontrol of the control circuit 210.

At a time t8, the data read on the basis of the first determinationlevel Vj1 and held in the sense latch circuit 120 are stored in the datalatch DL-L. The sense latch circuit 120 is cleared.

At a time t9, a read operation from the memory cells MC1 n 0 to MC1 n 7etc. is started on the basis of the third determination level Vj3, andthe read data are stored in the sense latch circuit 120.

On the basis of the read data from the memory cells MC1 n 0 to MC1 n 7etc. based on the third determination level Vj3, the read dataconversion circuit 220 performs an operation between the data held inthe sense latch circuit 120 and the data held in the data latch circuitDL-L under control of the control circuit 210 and changes bit data heldin the data latch DL-L to “1” only where both bit data are “0”.

At a time t11, a read operation is started from the memory cells MC2 n 0to MC2 n 7 etc. on the basis of the first determination level V1. At atime t12, output of the read data from the memory cells MC1 n 0 to Mc1 n7 etc. for a quarter sector held in the data latch DL-L is started undercontrol of the control circuit 210.

At a time t13, the data read on the basis of the first determinationlevel Vj1 and held in the sense latch circuit 120 are stored in the datalatch DL-R. The sense latch circuit 120 is cleared.

At a time t14, a read operation from the memory cells MC2 n 0 to MC2 n 7etc. is started on the basis of the third determination level Vj3 andthe read data are stored in the sense latch circuit 120.

On the basis of the data read from the memory cells MC2 n 0 to MC2 n 7etc. on the basis of the third determination level Vj3, the read dataconversion circuit 220 performs an operation between the data held inthe sense latch circuit 120 and the data held in the data latch circuitDL-R under control of the control circuit 210 and changes bit data heldin the data latch DL-R to “1” only where both bit data are “0” at a timet15.

At a time t16, output of data for a quarter sector held in the datalatch DL-R is started. At a time t17, output of data for one sector iscompleted.

While the data strings read at the times t1 are t3 are output, the readoperations of the data strings to be subsequently output are completedas hereinabove described. Thus, a delay time can be reduced whenoutputting read data from four-valued memory cells.

Third Embodiment

An exemplary data write operation in the structure of the flash memory1000 according to the first embodiment is now described as a thirdembodiment of the present invention.

FIGS. 31 to 36, illustrating operations of the third embodiment of thepresent invention, are conceptual diagrams showing data held in datalatches DL-L and DL-R and a sense latch SL as well as thresholds ofmemory cells and determination levels in writing in first to thirdprocessing steps of a write operation.

FIG. 31 shows data held in the respective latches in the firstprocessing step of the write operation, and FIG. 32 shows thresholds ofthe memory cells and determination levels in the first processing stepof the write operation.

FIG. 33 shows data held in the respective latches in the secondprocessing step of the write operation, and FIG. 34 shows thresholds ofthe memory cells and determination levels in the second processing stepof the write operation.

FIG. 35 shows data held in the respective latches in the thirdprocessing step of the write operation, and FIG. 36 shows thresholds ofthe memory cells and determination levels in the third processing stepof the write operation.

In a memory cell block subjected to the write operation, all memorycells are set to thresholds corresponding to data “11”.

Referring to FIGS. 31 and 32, input data (for a half sector: C9 h inhexadecimal notation) are stored in the data latch DL-L and transferredto the sense latch SL in the first processing step of the writeoperation. Writing up to a threshold corresponding to level 3 isperformed on the basis of a second determination level Vj2. During thewrite operation at the second determination level Vj2, data for theremaining half sector (93 h in hexadecimal notation) are stored in thedata latch DL-R.

Referring to FIGS. 33 and 34, a write data conversion circuit 230performs an operation on a corresponding set of bit data as to the datacontained in the data latches DL-R and DL-L thereby zeroing only bitdata of the sense latch SL corresponding to such data that bits held inthe data latch DL-L are “0” and bits held in the data latch DL-R are “1”in the second processing step of the write operation.

As shown in FIG. 33, the sense latch SL holds data “10110111” after thisoperation. On the basis of the data thus held in the sense latch SL,data are written in memory cells MC0 to MC7 corresponding to the bits ofthe sense latch SL respectively. The memory cells MC0 to MC7 areconnected to the same word line WL. A third determination level Vj3 isemployed as the determination value for a verify operation.

At this time, the data are written in memory cells corresponding to data“0” in the sense latch SL. Therefore, data (corresponding to data “01”)of level 4 are written in the memory cells MC1 and MC4 corresponding tothe first and fourth bits of the sense latch SL respectively.

The data are written through an FN (Fowler-Nordheim) tunnel current byapplying a high voltage to the word line WL.

A voltage below the word line voltage is applied to a bit line BLcorresponding to bits of bit data “1” in the sense latch SL, in order torelax a voltage applied from the word line WL. Consequently, data arewritten in only memory cells connected to a bit line BL corresponding tobit data “0” held in the sense latch SL.

Referring to FIGS. 35 and 36, the write data conversion circuit 230performs an operation of the data held in the data latches DL-R and DL-Rin the third step of the write operation, so that “0” is written in bitsof the sense latch SL corresponding to such a set of data that the bitdata held in the data latch DL-L are “1” and the bit data held in thedata latch DL-R are “0”. The determination value for the verifyoperation is changed to a first determination level Vj1, and data arewritten only in the memory cells MC3 and MC6.

As clearly understood from the above description, data are temporarilywritten as the level 3 also in the memory cells to be essentiallysubjected to data writing as level 4 in the step shown in FIGS. 31 and32. All memory cells subjected to writing as the level 4 are included inthe memory cells subjected to writing as the level 3 in FIGS. 31 and 32.

Similarly, memory cells subjected to writing as level 2 are included inmemory cells having thresholds of level 1 immediately before this step.

In other words, it follows that the memory cells subjected to writing oflevels 4 and 3 and those subjected to writing of levels 2 and 1 areseparated when performing the writing shown in FIGS. 31 and 32.

In the four-valued flash memory according to the third embodiment,therefore, the write operation is already started when data for a halfsector are input in the data latch DL-R after input of a write command,and hence the write time can be reduced.

The effect of such reduction is particularly remarkable when thecondition of the following expression (2) is satisfied:{sector size (byte)×(input time per byte)×½}≧{write time of level3}  (2)

FIG. 37 is a timing chart for illustrating the write operation of theflash memory according to the third embodiment.

Referring to FIG. 37, input of data (data of upper bits) for a halfsector is started at a time t1, and storage in the data latch DL-L isstarted from a time t2.

When data input for the front half sector is terminated, the data storedin the data latch DL-L are transferred to the sense latch SL at a timet3.

Input of data (data of lower bits) for a half sector is started at thetime t3, and storage in the data latch DL-R is started from a time t4.

At a time t5, a write operation is performed in response to the seconddetermination level Vj2 on the basis of the data stored in the senselatch SL.

When the write operation at the second determination level Vj2 isterminated at a time t6, the sense latch SL is cleared.

The write data conversion circuit 230 starts an operation on the datacontained in the data latches DL-R and DL-L as to a corresponding set ofdata at a time t7, and zeros only bit data of the sense latch SLcorresponding to such data that bits held in the data latch DL-L are “0”and bits held in the data latch DL-R are “1” from a time t8.

At a time t9, a write operation is performed in response to the thirddetermination level Vj3 on the basis of the data stored in the senselatch SL.

When the write operation at the third determination level Vj3 isterminated at a time t10, the sense latch SL is cleared.

The write data conversion circuit 230 starts an operation of the dataheld in the data latches DL-R and DL-L at a time till, and writes “0” inbits of the sense latch SL corresponding to such a set of data that bitdata held in the data latch DL-L are “1” and bit data held in the datalatch DL-R are “0” from a time t12.

At a time t13, a write operation is performed in response to the firstdetermination level Vj1 on the basis of the data stored in the senselatch SL.

While the data string input at the time t1 is written, storage of thedata string to be subsequently written is completed as hereinabovedescribed. Thus, a delay time can be reduced when writing data infour-valued memory cells.

[Modification of Third Embodiment]

In the write operation according to the third embodiment, reduction ofthe write time is enabled. On the other hand, the number of data latchesemployed in the write operation can be reduced.

FIGS. 38 and 39, illustrating operations of a modification of the thirdembodiment of the present invention, are conceptual diagrams showingdata held in the data latches DL-L and DL-R and the sense latch SL aswell as thresholds of the memory cells and determination levels inreading in a second processing step of a write operation.

FIG. 38 shows data held in the respective latches in the secondprocessing step of the write operation, and FIG. 39 shows thresholds ofthe memory cells and determination levels in the second processing stepof the write operation.

First, data are written in memory cells in a first processing step ofthe write operation, similarly to FIGS. 31 and 32.

Then, data of a remaining half sector are captured in the data latchDL-L again in the second processing step of the write operation. On theother hand, the data written in the memory cells in the first processingstep are read at the second determination level Vj2 and stored in thesense latch SL. The write data conversion circuit 230 performs anoperation similar to that in FIGS. 33 and 34 between the data held inthe data latch DL-L and the data held in the sense latch SL, andrewrites the contents of the sense latch SL. A write operationresponsive to the third determination level Vj3 is executed on the basisof the data in the sense latch SL.

Then, the data written in the memory cells are read at the seconddetermination level Vj2 again and stored in the sense latch SL. Thewrite data conversion circuit 230 performs an operation similar to thatin FIGS. 35 and 36 between the data held in the data latch DL-L and thedata held in the sense latch SL, and rewrites the contents of the senselatch SL. A write operation responsive to the first determination levelVj1 is executed on the basis of the data in the sense latch SL.

The number of data latches necessary for data writing can be reduced dueto the aforementioned operations.

Fourth Embodiment

A fourth embodiment of the present invention provides write processingcapable of simultaneously performing writing at the determination levelVj3 described with reference to FIGS. 33 and 34 and writing at thedetermination level Vj1 described with reference to FIGS. 35 and 36 withreference to the third embodiment, in order to further reduce the timerequired for the write operation in the third embodiment.

In the write processing of the fourth embodiment, write inhibit voltagesare applied to drains of memory cells not subjected to data writing whenwriting data in threshold levels 4, 3 and 2 at determination levels Vj3,Vj2 and Vj1, as shown in Table 1. TABLE 1 Gate Voltage (Word Line SourceWrite Mode Voltage) Drain Voltage Voltage Writing at write bit VW4(e.g., 17 V) V1 (e.g., 0 V) open Level 4 write inhibit VW4 (e.g., 17 V)V3 (e.g., 6 V) open bit Writing at write bit VW3 (e.g., 16 V) V1 (e.g.,0 V) open Level 3 write inhibit VW3 (e.g., 16 V) V3 (e.g., 6 V) open bitWriting at write bit VW2 (e.g., 15 V) V1 (e.g., 0 V) open Level 2 writeinhibit VW2 (e.g., 15 V) V3 (e.g., 6 V) open bit

FIGS. 40 and 41, illustrating operations of the fourth embodiment of thepresent invention, are conceptual diagrams showing data held in datalatches DL-L and DL-R and a sense latch SL as well as thresholds ofmemory cells and determination levels in writing in a second processingstep of a read operation.

FIG. 40 shows data held in the respective latches in the secondprocessing step of the write operation, and FIG. 41 shows thresholds ofthe memory cells and determination levels in the second processing stepof the write operation.

First, data are written in memory cells on the basis of a seconddetermination level Vj2 in a first processing step of the writeoperation, similarly to FIGS. 31 and 32.

Then, data for a remaining half sector are captured in the data latchDL-R in the second processing step of the write operation. A read dataconversion circuit 230 performs an operation as to the data contained inthe data latches DL-R and DL-L on a corresponding set of bit data,thereby zeroing corresponding bit data of the sense latch SL only whenbit data held in the data latch DL-L and the bit data held in the datalatch DL-R are different from each other. This corresponds to inversionof results of an exclusive OR operation of the respective bit data heldin the data latches DL-R and DL-L.

On the basis of the values of the sense latch SL rewritten in theaforementioned manner, data are written in memory cells to satisfyconditions in Table 2. In the memory cells subjected to data writing,drain voltages are changed as shown in Table 2 in response to whetherthe written data are “01” or “10”. TABLE 2 Gate Voltage (Word Line DrainSource Write Mode Voltage) Voltage Voltage Writing at write bit VW4 V1open Level 4 sense latch: “0” (e.g., 17 V) (e.g., 0 V) data latch: “0”in DL-L, “1” in DL-R write inhibit bit VW4 V3 open (e.g., 17 V) (e.g., 6V) Writing at write bit VW4 V2 open Level 2 sense latch: “0” (e.g., 17V) (e.g., 2 V) data latch: “1” in DL-L, “0”, in DL-R write inhibit bitVW4 V3 open (e.g., 17 V) (e.g., 6 V)

The relation voltage V1<voltage V2<voltage V3 holds in Table 2. If setvalues of threshold voltages of memory cell transistors are different,the values of the voltages V2 and V3 may be changed while maintainingthe aforementioned relation between the voltages V1, V2 and V3.

The time required for data writing can be further reduced due to theaforementioned operations.

Fifth Embodiment

The aforementioned description has been made on data read and writeoperations with reference to data of two bits, i.e., four values held ina single memory cell.

Assuming that n≧m+1 and m≧0, where n and m represent natural numbers,one bit (e.g., the most significant bit) among n bits can be defined inreading at a 2^(n−1)-th determination level in n-bit data held in eachmemory cell when each memory cell holds data of 2^(n) values andconstant relation holds for association with write data and levels ofthresholds of memory cell transistors corresponding to the write data.

Further, another bit among n bits can be defined by read processing attwo determination levels, i.e., 2^(n−2)-th and (2^(n−1)+2^(n−2))-thdetermination levels.

In addition, still another bit among n bits can be defined by readprocessing at 2^(m) levels such as a Σ(2^(n−1−m+Y))-th determinationlevel (Σ: the sum from Y=0 to Y=m as to Y; m≧0 and n≧m+1).

Finally, a further bit among n bits can be defined by read processing atfirst, third, fifth, . . . , 2^(n−1)-th determination levels.

FIG. 42 illustrates association between write data allowing reading inthe aforementioned manner and threshold levels of memory celltransistors corresponding to the write data in the case of writing dataof three bits, i.e., eight values in a single memory cell.

The most significant bit is defined in reading at a fourth determinationlevel, an intermediate bit is defined in reading at second and sixthdetermination levels, and the least significant bit is defined inreading at first, third, fifth and seventh determination levels.

FIG. 43 illustrates association between write data allowing no readingin the aforementioned manner and threshold levels of memory celltransistors corresponding to the write data in the case of writing dataof three bits, i.e., eight values in a single memory cell.

Although the most significant bit is defined in reading at a fourthdetermination level, an intermediate bit is not defined in reading atsecond and sixth determination levels.

[Read Operation of 16-Valued Data]

A read operation is now described with reference to 16 (=2^(n); n=4)values. As described below, the flash memory according to thisembodiment is similar in structure to the flash memory 1000 according tothe first embodiment except that four data latch circuits DL-1 to DL-4are provided in place of the two data latch circuits DL-L and DL-R andthat a control circuit 210 performs different operations.

FIGS. 44 to 81, illustrating operations of the fifth embodiment of thepresent invention, are conceptual diagrams showing data held in the datalatches DL-1 to DL-4 and a sense latch SL as well as thresholds ofmemory cells and determination levels in reading in first to nineteenthprocessing steps of the read operation.

FIG. 44 shows data held in the respective latches in the firstprocessing step of the read operation, and FIG. 45 shows thresholds ofthe memory cells and determination levels in the first processing stepof the read operation.

Referring to FIGS. 44 and 45, data are collectively read from aplurality of memory cells for 1 KB, for example, connected to the sameword line at an eighth (=2^(n−1); n=4) determination level Vj8 andstored in a sense latch circuit 120 in the first processing step of theread operation.

FIG. 44 extractively shows a sense latch SL holding data for two bytesfirst output from a data input/output terminal group 10 in a readoperation for one sector in the sense latch circuit 120 and data latchesDL-1 to DL-4 corresponding thereto. Processing similar to that describedbelow is performed in parallel also on data read from the datainput/output terminal group 10 subsequently to the data for two bytes.

It is assumed that memory cells MC0 to MC15 holding a data stringcontaining data for one byte first output from the data input/outputterminal group 10 hold data “0111”, “0110”, “0100”, “0101”, “0001”,“0000”, “0010”, “0011”, “1011”, “1010”, “1000”, “1001”, “1101”, “1100”,“1110” and “1111” respectively.

It is also assumed that write data corresponding to thresholds of thememory cells from higher level 16 to lower level 1 respectively are“0111”, “0110”, “0100”, “0101”, “0001”, “0000”, “0010”, “0011”, “1011”,“1010”, “1000”, “1001”, “1101”, “1100”, “1110” and “1111”, as shown inFIG. 45.

Therefore, the sense latch circuit 120 holds data 00h and FFh inhexadecimal notation in an area for two bytes in FIG. 44.

The data collectively read at the eighth determination level Vj8 arestored in the data latch DL-1 from the sense latch SL. At this point oftime, the data stored in the data latch DL-1 are sequentially outputfrom the data input/output terminal group 10 byte by byte (or in 2-bytegroups) under control of the control circuit 210.

Thus, the read operation is first performed at the eighth determinationlevel Vj8 in the first processing step of the read operation since 16types of data in the memory cells are defined as “0” or “1” in readingat this level, as shown in FIG. 45. In other words, it follows that themost significant bits of the data stored in the memory cells MC0 to MC15are defined as “0” or “1”.

FIG. 46 shows data held in the respective latches in the secondprocessing step of the read operation, and FIG. 47 shows thresholds ofthe memory cells and determination levels in the second processing stepof the read operation.

Referring to FIGS. 46 and 47, a read operation is performed at a fourth(=2^(n−2); n=4) determination level in the second processing step of theread operation when the data are stored in the data latch DL-1 from thesense latch SL and the sense latch SL is cleared in the first processingstep, and the read data are stored in the sense latch circuit 120. Inother words, data are transferred from the sense latch SL to the datalatch DL-2 while data output from the data latch DL-1 is performed.After data transfer to the data latch DL-2, the sense latch circuit 120is cleared.

FIG. 48 shows data held in the respective latches in the thirdprocessing step of the read operation, and FIG. 49 shows thresholds ofthe memory cells and determination levels in the third processing stepof the read operation.

Referring to FIGS. 48 and 49, data are read at a twelfth(=2^(n−1)+2^(n−2); n=4) determination level Vj12 and stored in the senselatch circuit 120 in the third processing step of the read operation.The read data conversion circuit 220 changes data bits in the data latchDL2 in accordance with results of a NOR operation between inverted dataof the data held in the sense latch SL and the data held in the datalatch DL-2.

FIG. 50 shows data held in the respective latches in the fourthprocessing step of the read operation, and FIG. 51 shows thresholds ofthe memory cells and determination levels in the fourth processing stepof the read operation.

Referring to FIGS. 50 and 51, data are read at a second (=2^(n−3); n=4)determination level Vj2 in the fourth processing step of the readoperation when the data are stored in the data latch DL-1 from the senselatch SL and the sense latch SL is cleared in the third processing step,and the read data are stored in the sense latch circuit 120. Data areoutput from the data latch circuit DL-1 and thereafter data transferfrom the sense latch SL to the data latch DL-3 is performed while dataare output from the data latch circuit DL-2. After data transfer to thedata latch DL-3, the sense latch circuit 120 is cleared.

FIG. 52 shows data held in the respective latches in the fifthprocessing step of the read operation, and FIG. 53 shows thresholds ofthe memory cells and determination levels in the fifth processing stepof the read operation.

Referring to FIGS. 52 and 53, data are read at a sixth(=2^(n−2)+2^(n−3); n=4) determination level Vj6 and stored in the senselatch circuit 120 in the fifth processing step of the read operation.The read data conversion circuit 220 changes data bits in the data latchDL3 in accordance with results of a NOR operation between inverted dataof the data held in the sense latch SL and the data held in the datalatch DL-3.

FIG. 54 shows data held in the respective latches in the sixthprocessing step of the read operation, and FIG. 55 shows thresholds ofthe memory cells and determination levels in the sixth processing stepof the read operation.

Referring to FIGS. 54 and 55, data are read at a tenth(=2^(n−1)+2^(n−3); n=4) determination level Vj10 in the sixth processingstep of the read operation when the data are stored in the data latchDL-3 from the sense latch SL and the sense latch SL is cleared in thefifth processing step, and the read data are stored in the sense latchcircuit 120. Data are output from the data latch circuit DL-1, andthereafter data are transferred from the sense latch SL to the datalatch DL-4 while data are output from the data latch circuit DL-2. Afterdata transfer to the data latch DL-4, the sense latch circuit 120 iscleared.

FIG. 56 shows data held in the respective latches in the seventhprocessing step of the read operation, and FIG. 57 shows thresholds ofthe memory cells and determination levels in the seventh processing stepof the read operation.

Referring to FIGS. 56 and 57, data are read at a fourteenth(=2^(n−1)+2^(n−2)+2^(n−3); n=4) determination level Vj14 and stored inthe sense latch circuit 120 in the seventh processing step of the readoperation. The read data conversion circuit 220 changes data bits in thedata latch DL4 in accordance with results of a NOR operation betweeninverted data of the data held in the sense latch SL and the data heldin the data latch DL-4.

FIG. 58 shows data held in the respective latches in the eighthprocessing step of the read operation, and FIG. 59 shows thresholds ofthe memory cells and determination levels in the eighth processing stepof the read operation.

Referring to FIGS. 58 and 59, the read data conversion circuit 220performs an OR operation between the bit data held in the data latchesDL-3 and DL-4 and stores the results in the data latch circuit DL-3.

FIG. 60 shows data held in the respective latches in the ninthprocessing step of the read operation, and FIG. 61 shows thresholds ofthe memory cells and determination levels in the ninth processing stepof the read operation.

Referring to FIGS. 60 and 61, data are read at a first (=2^(n−4); n=4)determination level Vj1 and stored in the sense latch circuit 120 duringdata output from the data latch DL-2. Data in the sense latch SL aretransferred to the data latch DL-4. The read data conversion circuit 220outputs inverted data of data held in the data latch DL-3 to the datainput/output terminal group 10 after data output from the data latchDL-2 is terminated.

FIG. 62 shows data held in the respective latches in the tenthprocessing step of the read operation, and FIG. 63 shows thresholds ofthe memory cells and determination levels in the tenth processing stepof the read operation.

Referring to FIGS. 62 and 63, data are read at a third(=2^(n−3)+2^(n−4); n=4) determination level Vj3 and stored in the senselatch circuit 120 in the tenth processing step of the read operation.The read data conversion circuit 220 changes data bits in the data latchDL-4 in accordance with results of a NOR operation between inverted dataof data held in the sense latch SL and data held in the data latch DL-4.

FIG. 64 shows data held in the respective latches in the eleventhprocessing step of the read operation, and FIG. 65 shows thresholds ofthe memory cells and determination levels in the eleventh processingstep of the read operation.

Referring to FIGS. 64 and 65, data are read at a fifth(=2^(n−2)+2^(n−4); n=4) determination level Vj5 and stored in the senselatch circuit 120 during data output from the data latch DL-2. Data inthe sense latch SL are transferred to the data latch DL-1.

FIG. 66 shows data held in the respective latches in the twelfthprocessing step of the read operation, and FIG. 67 shows thresholds ofthe memory cells and determination levels in the twelfth processing stepof the read operation.

Referring to FIGS. 66 and 67, data are read at a seventh(=2^(n−2)+2^(n−3)+2^(n−4); n=4) determination level Vj7 and stored inthe sense latch circuit 120 in the twelfth step of the read operation.The read data conversion circuit 220 changes data bits in the data latchDL1 in accordance with results of a NOR operation between inverted dataof data held in the sense latch SL and data held in the data latch DL-1.

FIG. 68 shows data held in the respective latches in the thirteenthprocessing step of the read operation, and FIG. 69 shows thresholds ofthe memory cells and determination levels in the thirteenth processingstep of the read operation.

Referring to FIGS. 68 and 69, the read data conversion circuit 220performs an OR operation between bit data held in the data latches DL-1and DL-4 and stores the results in the data latch DL-4.

FIG. 70 shows data held in the respective latches in the fourteenthprocessing step of the read operation, and FIG. 71 shows thresholds ofthe memory cells and determination levels in the fourteenth processingstep of the read operation.

Referring to FIGS. 70 and 71, data are read at a ninth(=2^(n−1)+2^(n−4); n=4) determination level Vj9 and stored in the senselatch circuit 120 during data output from the data latch DL-2 or DL-3.Data in the sense latch SL are transferred to the data latch DL-1.

FIG. 72 shows data held in the respective latches in the fifteenthprocessing step of the read operation, and FIG. 73 shows thresholds ofthe memory cells and determination levels in the fifteenth processingstep of the read operation.

Referring to FIGS. 72 and 73, data are read at an eleventh(=2^(n−1)+2^(n−3)+2^(n−4); n=4) determination level Vj11 and stored inthe sense latch circuit 120 in the fifteenth processing step of the readoperation. The read data conversion circuit 220 changes data bits in thedata latch DL-1 in accordance of results of a NOR operation betweeninverted data of data held in the sense latch SL and data held in thedata latch DL-1.

FIG. 74 shows data held in the respective latches in the sixteenthprocessing step of the read operation, and FIG. 75 shows thresholds ofthe memory cells and determination levels in the sixteenth processingstep of the read operation.

Referring to FIGS. 74 and 75, the read data conversion circuit 220performs an OR operation between bit data held in the data latches DL-1and DL-4 and stores the results in the data latch circuit DL-4.

FIG. 76 shows data held in the respective latches in the seventeenthprocessing step of the read operation, and FIG. 77 shows thresholds ofthe memory cells and determination levels in the seventeenth processingstep of the read operation.

Referring to FIGS. 76 and 77, data are read at a thirteenth(=2^(n−1)+2^(n−2)+2^(n−4); n=4) determination level Vj13 and stored inthe sense latch circuit 120 during data output from the data latch DL-2or DL-3. Data in the sense latch SL are transferred to the data latchDL-1.

FIG. 78 shows data held in the respective latches in the eighteenthprocessing step of the read operation, and FIG. 79 shows thresholds ofthe memory cells and determination levels in the eighteenth processingstep of the read operation.

Referring to FIGS. 78 and 79, data are read at a fifteenth(=2^(n−1)+2^(n−2)+2^(n−3)+2^(n−4); n=4) determination level Vj15 andstored in the sense latch circuit 120 in the eighteenth processing stepof the read operation. The read data conversion circuit 220 changes databits in the data latch DL-1 in accordance with results of a NORoperation between inverted data of data held in the sense latch SL anddata held in the data latch DL-1.

FIG. 80 shows data held in the respective latches in the nineteenthprocessing step of the read operation, and FIG. 81 shows thresholds ofthe memory cells and determination levels in the nineteenth processingstep of the read operation.

Referring to FIGS. 80 and 81, the read data conversion circuit 220performs an OR operation between bit data held in the data latches DL-1and DL-4 and stores the results in the data latch DL-4.

After data output from the data latch DL-3 is terminated, the controlcircuit 210 starts outputting inverted data of data held in the datalatch DL-4 to the input/output terminal group 10.

The data read time from memory cells capable of holding 16-valued datacan be reduced due to the aforementioned operations.

In the above description, the determination levels are paired from thelower level for inverting the upper level of each pair, NORing twodetermination results and finally ORing the results of each pair.However, the present invention is not restricted to this structure butthe determination levels may alternatively be paired from the higherlevel.

[Write Operation of 16-Valued Data]

A data write operation is now described.

FIGS. 82 to 87 are conceptual diagrams showing data held in the datalatches DL-1 to DL-4 and the sense latch SL as well as thresholds of thememory cells and determination levels in writing in first to seventhprocessing steps of the write operation.

FIG. 82 shows data held in the respective latches in the firstprocessing step of the write operation, and FIG. 83 shows thresholds ofthe memory cells and determination levels in the first processing stepof the write operation.

FIG. 84 shows data held in the respective latches in the second andthird processing steps of the write operation, and FIG. 85 showsthresholds of the memory cells and determination levels in the secondand third processing steps of the write operation.

FIG. 86 shows data held in the respective latches in the fourth toseventh processing steps of the write operation, and FIG. 87 showsthresholds of the memory cells and determination levels in the fourth toseventh processing steps of the write operation.

In a memory cell block subjected to the write operation, all memorycells are set to thresholds corresponding to data “1111”.

Referring to FIGS. 82 and 83, input data (00h and FFh in hexadecimalnotation) are stored in the data latch DL-1 and transferred to the senselatch SL in the first processing step of the write operation. Writing isperformed up to a threshold corresponding to level 9 on the basis of theeighth determination level Vj8. During the write operation at the eighthdetermination level Vj8, data for two bytes in the remaining data arefurther stored in the data latch DL-2.

Referring to FIGS. 84 and 85, a write data conversion circuit 230performs an operation on a corresponding set of bit data as to datacontained in the data latches DL-1 and DL-2 in the second processingstep of the write operation, thereby zeroing only bit data of the senselatch SL corresponding to such data that bits held in the data latchDL-1 are “1” and bits held in the data latch DL-2 are “0”.

Data are written in the memory cells MC0 to MC15 corresponding to therespective bits of the sense latch SL on the basis of the data held inthe sense latch SL in the aforementioned manner. The memory cells MC0 toMC15 are connected to the same word line WL. The fourth determinationlevel Vj4 is employed as the determination value for a verify operation,and writing is performed up to level 5 of the thresholds.

At this time, data are written in memory cells corresponding to data “0”in the sense latch SL.

The data are written through an FN (Fowler-Nordheim) tunnel current byapplying a high voltage to the word line WL.

A voltage below the word line voltage is applied to bit lines BLcorresponding to the bit data “1” of the sense latch SL, in order torelax the voltage applied from the word line WL. Consequently, data arewritten in only memory cells connected with bit lines BL correspondingto the bit data “0” held in the sense latch SL.

Referring to FIGS. 84 and 85, further, the write data conversion circuit230 performs an operation on a corresponding set of bit data as to datacontained in the data latches DL-1 and DL-2 in the third processing stepof the write operation, thereby zeroing only bit data of the sense latchSL corresponding to such data that bits held in the data latch DL-1 are“0” and bits held in the data latch DL-2 are “1”.

Data are written in the memory cells MC0 to MC15 corresponding to therespective bits of the sense latch SL on the basis of the data held inthe sense latch SL in the aforementioned manner. The twelfthdetermination level Vj12 is employed as the determination value for theverify operation, and writing is performed up to level 13 of thethresholds.

During the write operation at the twelfth determination level Vj12, datafor the remaining quarter sector are stored in the data latch DL-3.

Referring to FIGS. 86 and 87, the write data conversion circuit 230performs an operation on a corresponding set of bit data as to datacontained in the data latches DL-1, DL-2 and DL-3 in the fourthprocessing step of the write operation, thereby zeroing only bit data ofthe sense latch SL corresponding to such data that bits held in the datalatch DL-1 are “1”, bits held in the data latch DL-2 are “1” and bitsheld in the data latch DL-3 are “0”.

Data are written in the memory cells MC0 to MC15 corresponding to therespective bits of the sense latch SL on the basis of the data held inthe sense latch SL in the aforementioned manner. The seconddetermination level Vj2 is employed as the determination value for theverify operation, and writing is performed up to level 3 of thethresholds.

Referring to FIGS. 86 and 87, further, the write data conversion circuit230 performs an operation on a corresponding set of bit data as to datacontained in the data latches DL-1, DL-2 and DL-3 in the fifthprocessing step of the write operation, thereby zeroing only bit data ofthe sense latch SL corresponding to such data that bits held in the datalatch DL-1 are “1”, bits held in the data latch DL-2 are “0” and bitsheld in the data latch DL-3 are “1”.

Data are written in the memory cells MC0 to MC15 corresponding to therespective bits of the sense latch SL on the basis of the data held inthe sense latch SL in the aforementioned manner. The sixth determinationlevel Vj6 is employed as the determination value for the verifyoperation, and writing is performed up to level 7 of the thresholds.

Similarly, bit data of the sense latch SL corresponding to such datathat a set of bit data held in the data latches DL-1, DL-2 and DL-3correspond to (0, 0, 0) are zeroed. Then, the tenth determination levelVj10 is employed as the determination value for the verify operation,and writing is performed up to level 11 of the thresholds.

In the seventh processing step of the write operation, bit data of thesense latch SL corresponding to such data that a set of bit data held inthe data latches DL-1, DL-2 and DL-3 correspond to (0, 1, 1) are zeroed.Then, the fourteenth determination level Vj14 is employed as thedetermination value for the verify operation, and writing is performedup to level 15 of the thresholds. During the write operation up to level15, data of the remaining two bytes are further stored in the data latchDL-4.

In eighth to fifteenth processing steps (not shown) of the writeoperation, bit data of the sense latch SL corresponding to such datathat sets of bit data held in the data latches DL-1, DL-2, DL-3 and DL-4correspond to (1, 1, 1, 0), (1, 1, 0, 1), (1, 0, 0, 0), (1, 0, 1, 1),(0, 0, 1, 0), (0, 0, 0, 1), (0, 1, 0, 0) and (0, 1, 1, 1) are zeroed.Then, the first, third, fifth, seventh, ninth, eleventh, thirteenth andfifteenth determination levels are employed as the determination valuefor the verify operation, and writing is performed up to thresholdlevels corresponding to the respective steps.

In a 16-valued flash memory according to the fifth embodiment, ashereinabove described, the write operation is already started while dataare input in at least one of a plurality of data latch circuits afterinput of a write command, whereby the write time can be reduced.

Sixth Embodiment

While the above description has been made with reference to four datalatch circuits, three data latch circuits are sufficient for outputtingdata, NORing data and ORing bits respectively.

The flow of processing for writing data in 16-valued memory cells withthree data latch circuits DL-1 to DL-3 is now described.

FIGS. 88 to 117 are conceptual diagrams showing data held in the datalatches DL-1 to DL-3 and a sense latch SL as well as thresholds ofmemory cells and determination levels in writing in first to fifteenthprocessing steps of the write operation.

FIG. 88 shows data held in the respective latches in the firstprocessing step of the write operation, and FIG. 89 shows thresholds ofthe memory cells and determination levels in the first processing stepof the write operation.

In a memory cell block subjected to the write operation, all memorycells are set to thresholds corresponding to data “1111”.

Referring to FIGS. 88 and 89, input data (00h and FFh in hexadecimalnotation) are stored in the data latch DL-1 and transferred to the senselatch SL in the first processing step of the write operation. Writing isperformed up to a threshold corresponding to level 9 on the basis of aneighth determination level Vj8. During the write operation at the eighthdetermination level Vj8, data for two bytes in the remaining data arefurther stored in the data latch DL-2.

Data are read at the eighth determination level Vj8 again and stored inthe sense latch SL.

FIG. 90 shows data held in the respective latches in the secondprocessing step of the write operation, and FIG. 91 shows thresholds ofthe memory cells and determination levels in the second processing stepof the write operation.

Referring to FIGS. 90 and 91, a write data conversion circuit 230performs an operation on a corresponding set of bit data as to datacontained in the sense latch SL and the data latch DL-2 thereby zeroingonly bit data of the sense latch SL corresponding to such data that bitsheld in the sense latch SL are “1” and bits held in the data latch DL-2are “0”.

Data are written in memory cells MC0 to MC15 corresponding to therespective bits of the sense latch SL on the basis of the data held inthe sense latch SL in the aforementioned manner. A fourth determinationlevel Vj4 is employed as the determination value for a verify operation,and writing is performed from level 1 up to level 5 of the thresholds.

Referring to FIGS. 90 and 91, further, data are read at the eighthdetermination level Vj8 and stored in the sense latch SL also in thesecond processing step of the write operation. The write data conversioncircuit 230 performs an operation on a corresponding set of bit data asto data contained in the sense latch SL and the data latch DL-2 therebyzeroing only bit data of the sense latch SL corresponding to such datathat bits held in the sense latch SL are “0” and bits held in the datalatch DL-2 are “1”.

Data are written in the memory cells MC0 to MC15 corresponding to therespective bits of the sense latch SL on the basis of the data held inthe sense latch SL in the aforementioned manner in response to a twelfthdetermination level Vj12 from level 9 up to level 13 of the thresholds.

While data writing up to level 13 is performed, data of the remainingtwo bytes are further stored in the data latch DL-3.

In the aforementioned operations up to the second processing step of thewrite operation, reading at the determination level Vj8 may not beperformed each time but data may be held without clearing the data latchDL-1 for employing the data in the data latch DL-1 in place of the dataheld in the sense latch SL in the read operation. In the abovedescription, however, reading at the determination level Vj8 isperformed for matching with the following procedure.

FIG. 92 shows data held in the respective latches in the thirdprocessing step of the write operation, and FIG. 93 shows thresholds ofthe memory cells and determination levels in the third processing stepof the write operation.

Referring to FIGS. 92 and 93, data are read at the eighth determinationlevel Vj8 and stored in the sense latch SL in the third processing stepof the write operation. A control circuit 210 transfers inverted data ofthe data in the sense latch SL to the data latch DL-1.

FIG. 94 shows data held in the respective latches in the fourthprocessing step of the write operation, and FIG. 95 shows thresholds ofthe memory cells and determination levels in the fourth processing stepof the write operation.

In the fourth processing step of the write operation, data are read atthe fourth determination level Vj4 and stored in the sense latch SL.

The write data conversion circuit 230 stores results of an OR operationon a corresponding set of bit data as to data contained in the senselatch SL and the data latch DL-1 in the data latch DL1.

FIG. 96 shows data held in the respective latches in the fifthprocessing step of the write operation, and FIG. 97 shows thresholds ofthe memory cells and determination levels in the fifth processing stepof the write operation.

The write data conversion circuit 230 performs an operation on acorresponding set of bit data as to data contained in the data latchDL-3 and the sense latch SL for zeroing only bit data of the sense latchSL corresponding to such data that bits held in the data latch DL-3 are“1” and bits held in the sense latch SL are “1”.

Data are written in the memory cells MC0 to MC15 corresponding to therespective bits of the sense latch SL in level 1 to level 3 of thethresholds on the basis of the data held in the sense latch SL in theaforementioned manner while employing a second determination level Vj2for a verify voltage.

During writing at the second determination level Vj2, data of theremaining two bytes are stored in the data latch DL-2.

FIG. 98 shows data held in the respective latches in the sixthprocessing step of the write operation, and FIG. 99 shows thresholds ofthe memory cells and determination levels in the sixth processing stepof the write operation.

The write conversion data circuit 230 performs an operation on acorresponding set of bit data as to data contained in the data latchesDL-1 and DL-3 for zeroing only bit data of the sense latch SLcorresponding to such data that bits held in the data latch DL-1 are “0”and bits held in the data latch DL-3 are “1”.

Data are written in the memory cells MC0 to MC15 corresponding to therespective bits of the sense latch SL in level 5 up to level 7 of thethresholds on the basis of the data held in the sense latch SL in theaforementioned manner while employing a determination level Vj6 for theverify voltage.

FIG. 100 shows data held in the respective latches in the seventhprocessing step of the write operation, and FIG. 101 shows thresholds ofthe memory cells and determination levels in the seventh processing stepof the write operation.

In the seventh processing step of the write operation, data are read atthe eighth determination level Vj8 and stored in the sense latch SL, andthereafter the control circuit 210 transfers data from the sense latchSL to the data latch DL-1.

FIG. 102 shows data held in the respective latches in the eighthprocessing step of the write operation, and FIG. 103 shows thresholds ofthe memory cells and determination levels in the eighth processing stepof the write operation.

In the eighth processing step of the write operation, data are read at atwelfth determination level Vj12 and stored in the sense latch SL.

The write data conversion circuit 230 performs an OR operation on acorresponding set of bit data as to inverted data of data held in thesense latch SL and data contained in the data latch DL-1 and stores theresults of the operation in the data latch DL-1.

FIG. 104 shows data held in the respective latches in the ninthprocessing step of the write operation, and FIG. 105 shows thresholds ofthe memory cells and determination levels in the ninth processing stepof the write operation.

The write data conversion circuit 230 performs an operation on acorresponding set of bit data as to data contained in the data latchesDL-1 and DL-3 for zeroing only bit data of the sense latch SLcorresponding to such data that bits held in the data latch DL-1 are “0”and bits held in the data latch DL-3 are “0”.

Data are written in the memory cells MC0 to MC15 corresponding to therespective bits of the sense latch SL in level 9 up to level 11 of thethresholds on the basis of the data held in the sense latch SL in theaforementioned manner while employing a determination level Vj10 for theverify voltage.

FIG. 106 shows data held in the respective latches in the tenthprocessing step of the write operation, and FIG. 107 shows thresholds ofthe memory cells and determination levels in the tenth processing stepof the write operation.

In the tenth processing step of the write operation, data are read atthe twelfth determination level Vj12 and stored in the sense latch SL.

The write data conversion circuit 230 performs an operation on acorresponding set of bit data as to data contained in the sense latch SLand the data latch DL-3 for zeroing only bit data of the sense latch SLcorresponding to such data that bits held in the sense latch SL are “0”and bits held in the data latch DL-3 are “1”.

FIG. 108 shows data held in the respective latches in the eleventhprocessing step of the write operation, and FIG. 109 shows thresholds ofthe memory cells and determination levels in the eleventh processingstep of the write operation.

In the eleventh processing step of the write operation, data are writtenin the memory cells MC0 to MC15 corresponding to the respective bits ofthe sense latch SL in level 13 up to level 15 of the thresholds on thebasis of the data held in the sense latch SL while employing adetermination level Vj14 for the verify voltage.

FIG. 110 shows data held in the respective latches in the twelfthprocessing step of the write operation, and FIG. 111 shows thresholds ofthe memory cells and determination levels in the twelfth processing stepof the write operation.

In the twelfth step of the write operation, data are read at the seconddetermination level Vj2 and stored in the sense latch SL. The controlcircuit 210 transfers the data of the sense latch SL to the data latchDL1.

The write data conversion circuit 230 performs an operation on acorresponding set of bit data as to data contained in the sense latch SLand the data latch DL-2 for zeroing only bit data of the sense latch SLcorresponding to such data that bits held in the sense latch SL are “1”and bits held in the data latch DL-2 are “0”.

FIG. 112 shows data held in the respective latches in the thirteenthprocessing step of the write operation, and FIG. 113 shows thresholds ofthe memory cells and determination levels in the thirteenth processingstep of the write operation.

In the thirteenth processing step of the write operation, data arewritten in the memory cells MC0 to MC15 corresponding to the respectivebits of the sense latch SL from level 1 to level 2 of the thresholds onthe basis of the data held in the sense latch SL while employing adetermination level Vj1 for the verify voltage.

FIG. 114 shows data held in the respective latches in the fourteenthprocessing step of the write operation, and FIG. 115 shows thresholds ofthe memory cells and determination levels in the fourteenth processingstep of the write operation.

In the fourteenth step of the write operation, data are read at thefourth determination level Vj4 and stored in the sense latch SL.

The write data conversion circuit 230 performs an OR operation on acorresponding set of bit data as to inverted data of data held in thesense latch SL and data contained in the data latch DL-1 and stores theresults of the operation in the data latch DL-1.

FIG. 116 shows data held in the respective latches in the fifteenthprocessing step of the write operation, and FIG. 117 shows thresholds ofthe memory cells and determination levels in the fifteenth processingstep of the write operation.

The write data conversion circuit 230 performs an operation on acorresponding set of bit data as to data contained in the data latchesDL-1 and DL-2 for zeroing only bit data of the sense latch SLcorresponding to such data that bits held in the data latch DL-1 are “0”and bits held in the data latch DL-2 are “1”.

Data are written in the memory cells MC0 to MC15 corresponding to therespective bits of the sense latch SL from level 3 to level 4 of thethresholds on the basis of the data held in the sense latch SL in theaforementioned manner while employing a determination level Vj3 for theverify voltage.

Then, writing in levels 6, 8, 10, 12, 14 and 16 is performed in asimilar manner to the above.

In other words, writing is performed at a determination levelidentifiable in two values, and thereafter writing is performed atlevels identifiable in four values, eight values, . . . , 2 ^(n) valuesin the respective processing steps. In each processing step, reading isperformed by selecting two (one on upper or lower end) fromdetermination levels employed for previous writing (when changed fromfour-valued processing levels to eight-valued processing levels, forexample, three determination levels capable of determining four values)and bit data to be subjected to writing at the processing levels aredefined for performing writing.

In a 16-valued flash memory according to a sixth embodiment of thepresent invention, as hereinabove described, data writing from memorycells holding 16-valued data can be performed while reducing the numberof data latch circuits.

While data are written from the lower determination level in the abovedescription, data writing may alternatively be performed from the higherdetermination level.

Similarly to writing in the four-valued memory cells in the fourthembodiment, writing up to level 5 and level 13 of the thresholds,writing up to level 7 and level 15, writing up to level 2 and level 10,writing at the level 4 and up to the level 12, writing up to level 6 andlevel 14 and writing up to level 8 and level 16 can be simultaneouslyperformed. In such combinations, increased levels of the thresholdvoltages and level differences (threshold voltage differences) betweenthe respective sets are equal to each other and hence similar sets ofdrain voltages can be employed.

While the above description has been made with reference to memory cellsstoring 16-valued data, the present invention is more generallyapplicable to memory cells storing 2^(n)-valued data.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1-12. (canceled)
 13. A nonvolatile semiconductor memory devicecomprising: a memory cell array having a plurality of memory cellsarranged therein, each said memory cell including a storage element forholding binary data of 2 bits based on 2² threshold levels, wherein saidthreshold levels corresponds to data elements “11”, “10”, “00” and “01”forming a set of 2-bit data respectively in ascending order; a cellselection circuit collectively selecting a plurality of said memorycells from said memory cell array in response to an address signal; adata read circuit performing a read operation of storage data on saidselected plurality of memory cells on the basis of (2²−1) determinationlevels corresponding to boundaries between said threshold levels; and adata input/output circuit for transferring said storage data betweenoutside of said nonvolatile semiconductor memory device and said memorycells as binary data through k input/output nodes by every k bit (k:natural number), wherein storage data held in each said memory cell isgenerated from 2 bit data transferred at different timings through thesame said input/output node, wherein said data read circuit includes afirst read data hold circuit for holding first determination datadetermined at 2nd one of said determination levels to identify the dataof 2nd bit in said storage data held in said storage elements andsupplying said data to said data input/output circuit, and a second readdata hold circuit for holding second determination data determined at1st and 3rd ones of said determination levels to perform an operation onsaid second determination data for identification of the data of 1st bitin said storage data.